Watch Those Vices
Do you have a reputation for having the mouth of a sailor? Nothing wrong with that, but there's a time and a place for it -- and a first meeting with your boyfriend's parents isn't one of them. So watch your language. Wait until you get to know them better to find out what's acceptable. Some people have different levels of which words are OK and which aren't.
The same goes for smoking. Your boyfriend should let you know about his parents' stance. They might ban smoking indoors, but they're OK with you smoking out on their patio. If not, you can hold off getting your nicotine fix for a few hours. Remember that some people find smoking extremely distasteful, and it's best to adhere to their wishes when you're in their home.
If alcohol is served in your boyfriend's parents' home, limit yourself. When you're in a restaurant, don't order alcohol unless they do or you get some indication (from your boyfriend or otherwise) that it's cool. Avoid the temptation to drink in order to relax yourself, and this includes drinking before you meet them. Also, make sure you aren't hungover from the night before, because that's not a good look for anybody, either.