Drifting apart from my train-wreck of a friend?

I actually had this problem in high school, but to a lesser extent. My friend stopped caring about school and started doing drugs. What I was taught was to stop hanging out with that person so they know that you disapprove. This didn't work. Then I decided to just make an effort to hang out with him as much as possible just to get him away from that life. Eventually, he quit drugs and started working more on school (slightly). All it takes is a little effort on your part. Hang out with him as much as possible but with the ground rules of no drinking and no drugs. Be persistent. Make it more entertaining to be with you than drinking and doing drugs.