Sadly, the only way to find out what works for you is trial and error. Something that's great one year might be lousy a few years on. The diaphragm is just dandy when you are mighty and empowered and want to scare the boys with your knowledge and comfort in anatomy, but it's a real damn hassle if you like to indulge in rumpy-pumpy several times a day. (You lucky beast, you.) The Pill might serve you well for years, and then go haywire if you start travelling a lot. Condoms are just nifty if shagging is a rare event, but can seem all wrong if you are in a long term relationship (no reason why they should, though, but it's all down to perception). So, to expand a little on ymelup's original write up:

Barrier methods
  • Condoms
    The main cause of failure here is carelessness: fingernails or sharp jewellery puncturing or tearing the latex, oh, or not putting one on soon enough. No, scratch that, the biggest problem is that some men complain they can't feel anything when using a condom, and are therefore so unwilling to use them they'll do anything possible to avoid it. In a nicer world, though, the biggest problem, apart from the truly horrible taste they leave behind, is the strange sequence they can impose on sex. Condoms are far from perfect, but failure rates if they are used correctly are around one per cent, and they have the bonus of preventing the spread of STDs. In real life, failure rates are 2 to 4 per cent. Non-latex condoms are available for the allergic.

  • Diaphragm or cervical cap
    Once you have mastered the art of Zen and the art of diaphragm insertion, they are pretty effective. It's useful in that it doesn't have to be an interruption to sex, but you have to think about putting it in early so you don't have to vanish into the bathroom to struggle and curse at a crucial moment. Oh, and then you have to leave it in for hours afterwards. And if you want to in-and-out bit of sex more than once, you have to deal with those foul-tasting spermicidal creams and/or pessaries. Like condoms, it can enforce a particular sequence events into lovemaking. You will need to be fitted for a cap or diaphragm, and practice with it before relying on it. If you gain or lose more than about ten pounds, you'll need a new one. This is only appropriate for women entirely comfortable with dealing with their bits. Because of the hassle factor, effectiveness varies from 94 to 96 per cent. Which is on the scarily low side.

  • Female condom
    Instant hilarity. If you can avoid the one-size-fits-all discomfort, and the absurdly lound crinkling crunching crisp packet sounds, it's fine. But very expensive. Usage rates are too low to be entirely sure of failure rates, but probably in the 90-95 percent safe area.

  • Contraceptive Sponge
    Huge failure rate, even when correctly used and lathered up with spermicidal jelly. Only to be used if you are trying to reduce the chance of pregnancy, rather than determined to prevent it.

Surgical methods
  • Sterilisation
    For women, it's usually not a hysterectomy but a spot of fallopian tube tying. Very very hard to get a doctor to agree to this if you are still in your twenties. Unlike a vasectomy, it's almost impossible to reverse. Practically 100 per cent effective.

  • IUD or Coil
    The old fashioned ones are evil, vicious nasty things. They are intrusive and prone to causing serious infections. Usually only fitted to women who have already had children. That said, the new ones are more flexible and more adaptable to the body (made from copper beads or plastic) and less likely to case infection and they can have the added benefit of reducing heavy, painful periods. Some contain additional hormonal contraceptives but in general IUDs are not contraceptives--they work by irritating the lining of the womb and interupting a pregnancy at a very early stage by preventing a fertilised egg from taking up residence. 98 to over 99 per cent effective, depending on type of device.
Chemical methods
  • The Pill
    Mostly wonderful. It's effective IF you take it exactly as intended and don't mind living with your body thinking its pregnant. The smaller dose modern pills have to be taken at a precise time. Remembering this (especially if you move between different time zones on a regular basis) can be hell. Oh, and if you throw up or have an upset stomach, you're not safe against pregnancy for two weeks. During that time you'll have to use a barrier method. 99 per cent effective.

  • Depo Provera
    Marvellous stuff if it suits you. No more ovulation, and no bleeding at all after the first month or so. Four injections a year is very very low hassle. The spotting and potential weight gain is less attractive. Over 99 per cent effective.

  • Implant
    Five matchstick-sized wands inserted under the skin in the upper arm, giving slow release hormonal contraception. Lasts up to five years. Expensive, and if there are major side effects, can be hard to remove quickly. This has fallen out of vogue except in areas or situations where a single long-term solution to contraception is useful. 99 per cent effective in year one, 98 per cent effective over five years.
Other methods
  • Rhythm method
    Though popular with catholics, even with the modern, high-tech version that involves monitoring temperature and viscosity and so on, it's only more effective than jumping up and down three times and chanting Mary Had a Little Lamb in Latvian if you and your partner are extremely organised and disciplined. If you can put the effort into learning about your body and its responses, and if you can be very strict about abstaining as needed, then this can be as successful as many of the other 'good' methods, with a 97-98 per cent effective method. Otherwise you are looking at failure rates around that of the sponge (six to eight per cent). It is not appropriate at all if you have an irregular cycle.

    note: anthropod very rightly pulled me up on this flippant dismissal of something I know little about, and wrote fertility awareness which contains far more excellent information about basing conception and contraception on a knowledge of your own body than I could do justice to here.

  • Morning after pill
    Emergency contraception, if something fails (e.g. an exploding condom, or an outbreak of idiocy). Not to be relied on in general, but useful to prevent unwanted pregnancies. Morning after is a terrible misnomer: you actually have about 72 hours of safety in which to take this. It's just a few old-fashioned very high hormone dose pills. It does the trick. Doctors advise that you shouldn't resort to this more than three or four times.