Hi, my cat Tigger got a UTI. I've been doing some research online and found out that there are several types of crystals - struvite and oxalate being the main ones. According to this user on yahoo, depending on the type of treatment, you need to administer either Cranberry Juice or Apple Cider Vinegar.
"Cranberry extract acidifies the urine and lowers bacteria levels, and is useful if she [the cat] has struvite crystals. However, if oxalate crystals, you need to go alkaline. To alkalize, mix 1 tsp apple cider vinegar in 1/4 c water and administer at least a tablespoon of the mix as best you can. You can also mix a little baking soda in with food to help alkaline the system".
I took Tigger to the vet and found out he had struvite cystals - this meant he needed Cranberry juice. Was administering 3ml of undiluted cranberry juice to him every hour, and as he couldn't pee much initially, by the evening (after many hours), he could pee. He was peeing significant puddles :)
I urge you to find out exactly what type of crystals your cat has and then administer the proper treatment! Hope this helps.