Birth control has come a long way since its inception in classical times; the past few millennia have included innovative improvements on old favorites such as intrauterine devices, condoms, and hormonal treatments galore to enable people to sow their wild oats while ensuring a crop failure1. Most of these methods are temporary, operating under the assumption that while the users do not want children now, they will most probably want them later.
So what about those who are dead certain they will never want children? Or women for whom childbirth will almost certainly be fatal, and thus do not want to risk pregnancy of any sort? Or women who not wish to have an abortion or be be put into a position where they have to choose whether to have one or not? In these cases, barrier birth control is still not 100% effective, and hormonal birth control, while more effective than barrier methods if used stringently, still have a certain margin of error one must deal with. In addition, the trouble with hormonal birth control is that the cost adds up over the years one takes it, it affects the moods and hormonal balance of the one taking it, and not everyone uses it in the exactly ideal manner that is recommended. With this in mind, as well as a definite unwillingness to reproduce, many people opt to undergo a sterilization procedure to eliminate their chances of having children.
The sterilization procedures available to men, to date, are limited to vasectomy and castration; since the latter would render moot the reason for the procedure, as well as causing a hormonal disruption that would need to be remedied by pharmaceutical therapy, for obvious reasons it is not considered by most men. The options available to women are tubal ligation and hysterectomy, the latter of which is considered much too extreme for simple birth control. More recently, a procedure called "Essure" has been developed by a United States-based company, Conceptus, and offers a non-invasive, non-surgical method of sterilization wherein implants are inserted through the cervix into the Fallopian tubes, which cause the tubes to scar over gradually and become completely blocked in a few months.
If you are interested in obtaining permanent birth control, the search for a doctor to perform the procedure may be more painful than its aftereffects, especially if you are under 30 years of age and have not had children (and, in some cases, being female also makes you less likely to be taken seriously)2. Quite frequently, doctors will simply refuse to perform the procedure on someone fitting that description. Some of this reluctance stems from a father-knows-best view of the situation, or an assumption that no matter what the indivdual story of the client, the doctor knows what the client will regret better than the client him or herself. However, it certainly does not help matters if doctors have to field lawsuits from people who want children ten years after the procedure and realizes, to their horror, that they got exactly what they asked for when they had the procedure done.
Just a final note: though the above methods of sterilization are almost always completely effective and completely permanent, there is the odd time when, in the case of vasectomy and tubal ligation, the body somehow reconstructs enough of the severed tubes for fertility to be regained. In this case, really, there is no way of knowing until a missed period announces the presence of a new zygote in the lucky lady's body. However, the chances of this happening are extremely slim, and if one wishes to avoid any chances of spreading one's genetic material around, the above procedures are one's best bet. Also, sterilization methods offer zero protection versus sexually transmitted disease, so barrier protection is still necessary to reduce the risk of acquiring STDs--unless, of course, you trust your partner enough that you are willing to place your health in his or her hands.
Of course, if you're really squeamish about breeding, may I recommend the following: not bathing, avoiding your dentist and toothbrush, adopting several forms of bigotry, and foregoing normal speech for 1337-speak.
1 Shamelessly stolen from Terry Pratchett's book, Mort.
2 According to many testimonies from frustrated permanent birth control seekers on the 4,000+ member Childfree LiveJournal community.