The most important test is sending the sample to the lab. If the lab sample is clear, it sounds more like bladder irritation rather than true UTI. If the lab samples show actual recurrent UTI, then she should (NICE guidance) be having a DMSA scan to check out her kidneys.
If it's bladder irritation (and all these are also useful in kids with recurent UTI, as they reduce the odds of getting a UTI):
Drink loads, even when well. For an average sized 5 year old that should be a total fluid intake of nearly a litre 1/2 per day (more if big for age). The fluids should be spread out through the day - make sure she's not drinking very little when at school, then having most of what she does drink in a few hours in the evening.
She should be drinking enough to make her urine, when well, watery in colour - just a tinge of colour, no more. Concentrated urine is very irritating to the bladder, tube to the outside (urethra), and the delicate surrounding tissues. Irritated bladders make can mimic infection annoyingly well, but are also more prone to letting infection take off.
No fizzy drinks or fresh citrus drinks (basically, the acid load gets wee'd out, which irritates the bladder).
Make sure she isn't constipated. If a kid's got poo waiting to come out, then it makes it harder to empty the bladder, leading to stale urine being left behind, and also means more bugs floating around the perineum just waiting to cause trouble.
Hygiene isn't just about wiping the right way, though that is important. You've probably already banned bubblebath, but try to avoid using any products in the bath (at a push, use stuff marketed for people with eczema), and if you need to wash her hair in the bath, then do it last, so she isn't sitting in shampooish water. Irritating the urethra with bath products makes both UTI and bladder irritation more likely. Cotton knickers. If she tends to get any dampness at all, encourage her to be able to get clean dry knickers on at school.
Check how often she is actually going to the toilet for a wee. Hanging on leads to stale wee (back to irritating the bladder) and to leaks (irritating the urethra and surrounding area). Encourage her to sit properly on the toilet, not hover or rush off,to help with completely emptying her bladder.
Seriously, though, the single most common factor that makes a big difference is making certain they drink enough. As far as I can work out, at least 19 times out of 20 young kids drink way less at school than their parents think: there are just so many more exciting things for them to do! Worth keeping a daily diary of drinks for a bit? I've not met many kids that age who can reliably tell you what they normally drink at school, but most can manage to tell you what they had that day. Oh, and make sure a really good big drink at b'fast, as you can control that one...