There's hygiene things, of course, which your dr. has probably told you about, but just in case:
* Wipe front to back, never back to front. (Yes, I know women who were adults before they learned this.)
* Always pee after intercourse. (It's a good idea to pee before, too, but even more important after.)
* Don't get in the habit of "holding it" for a long time after you feel the urge to go.
* Wear underwear made of 100% cotton (or at least a 100% cotton crotch). (And, it goes without saying but I'll say it anyway... always wear underwear, and change it daily ;-).
Other than that... they say "cranberry juice," but actual cranberry juice is *nasty*. The drinkable stuff has a bunch of sugar or fruit juice added to it. The sugar may actually make you more prone to infection. If you can get ahold of some cranberry pills, taking them regularly may help. The reason cranberry juice helps is that it changes the pH of your urine, making it more acidic and therefore a place that bacteria are less inclined to grow.
Some women with chronic UTIs benefit from probiotic supplementation, too, but I'm way too wiped out to try to find my book and look up which kinds.