I was going to try to od on pills. The classic prescription sleeping pills and antidepressants. I am very glad I did my research first though and found that overdosing is actually pretty tricky to get right and can do some major non lethal damage when it isn't successful. The methods resource thread has a lot of information about a whole variety of methods.
Whatever you do, don't OD on paratecamol/acetaminophen (whatever it is called in America.) It is excruciatingly painful and you will wake up in in the E.R. with an antidote as the pain is literally like your liver being set on fire. If you want to OD, do it on something else.