Golf Digest has a short article about what to do if your boss asks you to play a round of golf. It's a pretty good article and echoes many points that I give in my book, On Course for Business--Women and Golf.
I'd like to add a couple of points. First, on that day, you should not be trying to shoot your personal best. If it happens that you play your best round, great (and be gracious about it)! But you shouldn't be so focused on your game that you go into a competitive mode. For me, that means I don't talk as much, and spend a bit more time on reads, etc. If you're playing with your boss, you want to be conversational as much as he or she is, and keep the atmosphere in your group light.
Second, make sure you're on your best behavior. It's not a day to drink alcohol, smoke your cigar, tell jokes, swear, or throw clubs, even if your boss is doing so. Imagine that you're playing golf with your priest or mother that day. Whoever it is that if you did something out of line, you know you'll never get away with it. If your boss is drinking alcohol, and offers you a drink, consider sipping it slowly. Or do what my niece does in college -- she pours some of her drink out when no one is looking.
Playing golf is a great way to deepen your relationship with your boss, but it can backfire. Believe me when I say I've heard stories that my audience members have shared about incredibly stupid behavior with bosses and clients.
P.S. Gents, don't let my book title scare you away from it. I have a chapter in it just for you. I talk about what to do if you're playing with a female business decision-maker. You might change your game a bit when playing with her.