Spell it out very clearly, such as not to be done inside or outside, at any time, by anyone- resident or visitor.

Make it so nobody can possibly claim they misunderstood something.

For instance, many will say they will smoke only outside, or only in their car. It's not true. And, even if they did, the smell comes in with them.

I issue pre screen questions and a notice that my units are No Smoking. I have it on my lease - No Smoking.

Putting it on the lease is one thing enforcing it is another.

As others have noted - tenants will cheat and when they go outside to smoke it stays on their clothing.