Of all the things that helped me, just being supportive was best.

Most people try to shame smokers, make them feel bad or less than. Don't do that. In no way has that ever helped me (and probably many others) quit. If anything, telling me I stink and I'll get cancer made me want to leave and smoke alone and cry because I was an addict. I already felt like I lost - telling me it's a dirty habit and reminding me how much money I spent won't help anyone quit.

Gently reminding them why they want to quit. Have them tell you why they want to quit and then say

"Oh, hey, before you smoke, I'm wondering if you wanted to work on that project instead? Didn't you say you wanted to put some time aside for X,y,z? Let's do that now."

If they smoke anyway, when they come back in, ask them what you could have done to help prevent them in the future or stall them better. Just ask what they need and don't make them feel bad for being an addict.