Be clear about what you expect. Keep reinforcing your expectations for your kids: doing well in classes at college, being a good employee, etc. Understanding your desire to see them succeed can help them make positive decisions to avoid alcohol and drugs.
Set goals. Young adults are reaching an age where their future is truly in sight. By talking about where they see themselves in the next year or two and how to get there you can encourage them to stay proactive about achieving their goals.
Reiterate rules. If your young adult is living on their own, or at college, you can still make it clear that alcohol and drugs are off-limits at home and around you. Make it clear that they need to respect you as a parent or caregiver, and that they have to respect your home and your space.
There's still the law. Underage drinking and the illegal use and possession of drugs are against the law. Remind your children that, even when they might no longer have to obey you, the law establishes rules of behavior they still need to follow.
You can encourage them to stay proactive about achieving their goals.