Cat has possible UI? any ideas about what i could feed her to help w this?

shes been to the vet several times over the past 4 monthes and nothing was detected by the vet and i cant/wont take her again anytime soon unless i HAVE TO! shes a 6 year old spayed female and what are some things i could change in her diet to help w her possible UI? CANNED FOOD isnt helping! (or does it) it just seems to give her the runs..