Yeah it's good to know yourself.
I noticed I do have a pretty good willpower, but possibly even more important are responsabilities (school) and friends who don't use anything illegal (going out with them results in only taking alcohol which is a harddrug too but I don't feel the inclination to use it all the time). If I would be sitting on my room every day for half of the day and be bored to death, I would be using more too. But now I come back from school, eat, go out with friends or watch a movie or something. I don't have time to use lots of stuff and if I do have a spare evening, I have to take into account my productivity of the next day.
A good prevention is only having a limited amount of everything available and limit your purchases. By that way you have a good reality check: "oh damn, I used all that in only one week?". Once it's out, it's out. Having large quantities around makes it easier to end up in a full-weekend binge with bad results (LOTS of examples on forums of people who bought a large stash "to last months" and used everything in two weeks)
Cocaine is a tricky one. I never used it and I'm kind of afraid too to try it, although I would really want to. I'm afraid that if someone would offer me a line while I'm a bit drunk, I wouldn't hesitate long. However, I know a lot of people who have tried it a couple of times and liked it a lot, but never got addicted to it. It's something special like for on a party or NYE, not for daily use. It's terribly expensive too.
Ketamine is something like that too. After having some good trips I had the urge to do it again (even dreamt about it once) but that subsides after a couple of weeks. Also the tolerance is a factor: once you notice you have to use more to get the same effect, you're using too much. Tolerance is bad (except for alcohol, where it improves your street cred.
).
Heroin, other strong opiates and crack (and other heavy stuff) I wouldn't try. Maybe I could resist myself of doing it again but I'm quite sure that ultimate rush will keep nagging in the back of your head for the rest of your life and makes everything else feel dull and boring. Some people are saying that of MDMA too, but I think the effect, when dosed carefully, is quite subtle and actually makes you appreciate "sober" experiences more too.
Very different from the huge heroin rush which makes you feel like being in your mothers womb while being king of the universe too.
Some drugs aren't addictive at all: LSD and shrooms have such a profound and long effect (especially lsd) and have a quick tolerance buildup so abuse is very uncommon. An LSD or shrooms trip is quite exhausting too and not always "pleasant" (but always interesting).
Baseline is, as always: know your substance and know yourself. Know your friends too, as they can have a big influence on your time passing and drug use.