What are some tips for convincing a good friend to stop smoking?

Probably just stop. There are many ways to do this and to fail at doing this, but the most common way people seem to quit is to just stop on your own. Apparently, up to three-quarters of ex-smokers of any substance report having quit without assistance ("cold turkey" or cut down then quit), and cessation without professional support or medication may be the most common method used by ex-smokers.
Quit contract. Smokers are very good at fooling themselves into smoking again. With a quit contract, the smoker has to answer to you, or to a group of friends, or to a group of people who are also trying to quit. Answering to others for one's indiscretions is difficult, and can help the quitter in his/her attempt.