STDs and hormonal imbalances (including those caused by pregnancy and breastfeeding) can make a person prone to urinary tract infections. If neither of these is an issue, you're one of the unfortunate few who have no known reason for UTIs. You could try drinking cranberry juice on a daily basis. I've heard that cranberry extract pills give you the same result. Or you can begin eating yogurt high in probiotics to help solve your problem.
Probiotics are also available in foods and dietary supplements (for example, capsules, tablets, and powders) and in some other forms as well. Examples of foods containing probiotics are yogurt, fermented and unfermented milk, miso, tempeh, and some juices and soy beverages.
I hope your health issue is resolved soon.