I must also add to all of those very well researched comments, that any
sex manual will recommend
combining two of the methods, doing prior research that they work well together.
For example, combine the pill with a condom, those are the two most effective methods (save a sterilization or such), or at least use spermocide with the pill. This is the best way, because the chances or getting pregnant between the two are next to none.
I used this method ever since I started having sex, though recently I went off the pill for personal reasons, so now me and my boyfriend use condoms along with spermocide. It's less effective, but it's something I am doing with the man I love, and we are able to handle the consequences if the condom rips, or anything like that.
Not being on the pill, I would stay away from casual sex, even though I always used condoms.
Some methods cannot be mixed, for example, a diaphragm is not to be used with a condom. A latex condom hitting a latex diaphragm during intercourse can cause cracks in both.
As for the Morning After pill that so many have mentioned above...
IT IS NOT A METHOD OF CONTRICEPTION
Contraception is prevention BEFORE sex, the Morning After pill is an emergency treatment for cases when the condom rips and the person is not on the pill, and more importantly, rape victims, (if they so choose).
It screws up your body a lot greater than any regular birth control pill, and is very dangerous to use more than once every few months.
It should not be viewed as birth control.
True, non-of those methods are safe (I had 2 pregnancy scares while using spermocide condoms and the pill).. But neither is abstinence. If you do everything else, but don't have intercourse, there is still a chance one can get pregnant from a not very careful move or such, if some sperm that has not been exposed to oxygen, gets into the birth canal. Such freak cases tend to occur in the shower or a bath, if the male ejaculates into the water, and happens to be close to the female, there have been reported cases of conception.
People who had a vasectomies have been known to impregnate, and women with a hysterectomy have been known to get pregnant with a fallopian tube pregnancy, which is very dangerous.
So once you have sex, you have responsibility, you should know that to do if you DO get pregnant, before it happens. And if you are not ready to think about such things, you are not ready to have sex.