Abstinence
Let's just get this one out of the way. It literally leaves much be desired, and yet it remains the only choice offered to many teenagers in America in answer to their many questions about sex.
- "How can I have sex and keep my risk of STDs and pregnancy to a minimum?" - "Don't have sex."
- "Will using spermicidal foam also protect me from STDs?" - "Don't have sex."
- "How can I tell when a girl likes me?" - "Well, son, first, don't have sex. Second ... actually, that's it."
Pretty silly. Much like living in a bubble, technically you can do anything you want and remain "abstinent", as long as you "don't have sex". For the purpose of STD prevention, the effectiveness of abstinence on it's own is generally proportional to how strictly you interpret that phrase, but because you're starting from nothing, you can stretch it quite a ways with relatively low risk of STD and no risk of pregnancy. For example, in highshool we defined anything that we could do while keeping our blue jeans fully on and zipped (pants not shorts, and no hands in or on the pants) as "not sex". I remember several evenings where I and five of my friends piled onto my bed together to conduct some intensive studies of abstinence. As Myrkabah told me recently, "anyone who considers a lack of intercourse to be a serious barrier in nakedkinkyfuntime (sic) is most likely suffering from a very, very dire lack of imagination." Words to live by.
Lifelong Monogamy
Another perennial suggestion of puritans, this trades promiscuity for safety. Once again you're living in a bubble, but now there are two of you in there. Thus, though I'm sure the puritans wouldn't approve, you can try almost every nasty habit and kink you can imagine (and get your partner to agree to) and you're about as likely to get an STD as if you had both abstained. Not as easy as it sounds, as one or the other person tends to wander sooner or later, but still a good deal if you can find it and stick with it.
Complete and Honest Communication
If, like most of us living in the real world, you ever change sexual partners, this will be your first line of defense against STDs. Its difficulty depends on the character of you and your (potential) partner(s). If everyone is honest with everyone else about what they've done with whom, then everyone can make informed decisions about what they're (un)willing to do. This is an inverse form of the Prisoner's Dilemma, so if anyone isn't "honest and complete", everyone loses. Also not as easy as it sounds. You'll need to discuss the sexual history of you and your (potential) partner(s), the more in depth the better, including "cured" STDs just to be safe. For best results you'll want to be comfortable starting and leading that discussion. Even then, given the nature of people (see above) and STDs (see below), this option by itself may seem akin to bringing a knife to gunfight, but it's better than nothing.
Disease testing for yourself and partner(s)
Honest Communication helps but it's only as effective as the accuracy of the information each person has. STDs are some of the craftiest diseases around. Many cause few or no symptoms for months or years after infection, leaving their host plenty of time to get around and infect all their pillow pals. Enter modern science. Getting tested for regularly for STDs and demanding the same of your (potential) partner(s) vastly improves the quality of information available to all concerned. Still, some STDs, like HIV, can have latency periods up to several months, during which testing won't detect them, so the statement "I've tested clean" should be the start of the discussion not the end.
Look before you "sleep"
If you're just too eager to get it on to stop and ask for an STD test, or if you have reason to doubt your partner's word when they say they're clean as the driven snow, your best choice (health-wise) is to not "sleep" with them. If you decide to go for it anyway, you can slightly reduce your risk by learning the possible visible symptoms for various STDs, and examining your prospective partner(s) for any of those symptoms. You can even do it playfully as part of undressing and caressing - the more thorough you are the better your chances. Though probably the most fallible way to check for STDs (see above about lack of symptoms), it may still keep you from playing with individuals who are obviously infectious.
Latex barriers
Those latex gloves doctors use aren't just for show - they form a non-porous barrier through which pathogens can't pass (in either direction). What works in the operating room can work just as well in the bedroom. As long as each person and their fluids remain on their own side of the barrier, latex gloves, condoms, and dental dams (for oral-vaginal intercourse) are very effective. For those with latex allergies, vinyl alternatives are also readily available. In a pinch, you can even use Saran Wrap ("non-microwavable", because the "microwaveable" stuff is porous) in place of dental dams (but not condoms or gloves - too delicate).
At this point, you might be wondering how all this latex and vinyl differs from wearing an encounter suit to bed. Well, modern barrier methods are considerably more responsive than an encounter suit, making for a much more pleasurable experience... Unless you (or your partner) have a kink for Darth Vader sex, in which case the suit might be a good investment. Also unlike an encounter suit, fluids can easily splash (or smear) beyond the edges of these barriers onto nearby skin. This still keeps your fluids separate, protecting you from STDs like HIV and syphilis, but contact STDs like herpes or genital warts will still go town on you. If you're shooting for your best odds of not getting an STD, "I've got a condom," should, like "I've tested clean", be the launching point for a more extensive discussion.
For those about to kink, keep it clean
Whatever your level of kink, from thinking vinyl pants are sexy to wanting to tie someone up and paddle them delirious, clean your props after each use. How to do this depends on the item in question. For vinyl (including vinyl pants), latex, or other non-porous materials this means hot water and soap. With silicon (also non-porous) you can even sterilize it by dropping it in boiling water for 3-5 minutes. On the other hand, any porous material, such as leather or rubber, will be very difficult to properly clean let alone sterilize, so keep them away from splatter zones or cover them with a disposable non-porous material (i.e., a glove or plastic wrap).
If your kink extends to piercing, cutting, or blood sport in general (giving or getting), treat it just like surgery: use latex gloves, sterile implements, and clean and disinfect the skin involved before starting. For the amateur, disposable single-use implements and sterilizing pads, which come sealed in individual sterile pouches, are safest. If you're on the receiving end of these activities, unless you saw them come out of a sterile sealed package or an autoclave, the implements might as well have just come out of someone else's body.
Given the choice, avoid all activities which might involve blood to blood contact
Most STDs are called such because their primary (often only) method of transmission are via bodily fluids such as saliva, vaginal secretions, ejaculate, and blood. Blood in particular offers a perfect living and transmission environment for most pathogens including STDs. Any time you expose your bloodstream to someone else's blood you're literally rolling out the red carpet for every disease they have: no one action is more dangerous.
So just don't do it! Don't do illegal IV drugs! If you do IV drugs, don't share needles with anyone! This solution seems pretty straightforward to me, but it's escaped millions of people the world over. Don't be one of them.
Blood transfusions also pose some degree of risk and should be avoided if possible. Although well screened, no whole blood can be guaranteed pathogen free. If you have a planned surgery coming up, some hospitals will allow you to have your own blood drawn and stored for transfusion during the surgery. Depending on your jurisdiction, a sterile synthetic blood substitute might also be available. More often the need for transfusion will arise from an accident or unforeseen illness, and your choice (if you're conscious to make it) will be rather simple - transfusion or death.
Take your time, Use Your Brain and Be Consistent
These three go hand-in-hand and I can't stress them enough. The true foundation of any endeavor to not get an STD, taking a few seconds to use your brain can keep you from doing things that will get you killed. Choosing not to shoot heroin for the first time on a dare, would be an excellent example of using your brain. But inconsistency, which generally springs from not using your brain, can completely sabotage any of the previous choices including abstinence. I knew a girl who was "abstinent" except for giving blow jobs, for which she didn't even use condoms because "technically blow jobs don't count". D'Oh! Diseases don't care about technicalities and they don't care about your rules!
Many states have a waiting period for guns, why don't you establish a waiting period for sex? While that means you can't spontaneously go at it with someone, it also means you'll have time use your brain instead of your hormones to decide. It also gives you time to get STD tests, make sure you have condoms, ask questions like "Who have you had sex with in the last three months?", and even buy some nice candles to make the eventual evening more romantic.
Parties, especially ones with an overabundance of alcohol and/or drugs, seem to be the "spontaneous" breaking point for many STD avoidance plans. In the words of an inebriated friend of mine in high school, "But what's the point of, like, living, if I can't get drunk and get it on with a cute chick?" In his case, complete STD avoidance seemed untenable, so I suggested he bring a couple of condoms to parties and use them - at least then he wouldn't get anything that would kill him. If he'd been willing to stick to only making out, his odds would have improved significantly (clothing also makes a good "barrier method"), but he wouldn't hear of it. There's no cure for willful stupidity.
As I said to begin with, anything short of living in a bubble constitutes gambling. That's life, but whether you want a reputation as an ice queen (or king) or a reputation that invites comparison with Debbie does Dallas, not having an STD prevention plan is a sucker's bet. Explore all your options while in a clear state of mind, decide what level of risk you're willing to accept, listen to your gut when it tells you to pass, choose how to limit your risk to that level (including combinations of the choices listed here), and figure out how to do that consistently and without fail. That way even if you end up losing, you'll be less likely to lose big, or at the very least you won't end up wondering how it happened.