D-Mannose is a sugar (looks like powdered sugar) that you mix with water and drink. The theory is that the sugar binds to the bacteria and you "flush" the baddies out of your system.
I too got about 5 UTI's a year until I started using it. You can use it as a preventative a couple of times a week, or in larger doses when you have the backache, heavy urine, etc. and know a UTI's coming on. It will help A LOT with an active UTI, if not make it go away all together. In any case, I found the constant antibiotic treatment to make the UTI's keep coming back in cycles and was sure that they were compromising my immunity in general. Even one of my MD's agreed that they're just not ideal for UTI's.
I take it after sex, and additionally about once a week just as a preventative, and haven't had a UTI in months. What a relief!
The D-Mannose can be a little expensive, but still cheaper than antibiotics. And no side effects that I've heard of or experienced.