Poor personal hygiene is frequently associated with a sensory processing issue (or sensory over-excitabilities, however you choose to look at it) and/or with poor executive functioning (they mean to do these things, but forget to do them or "run out of time" repeatedly, so days go by). Identifying what is driving or perpetuating the behavior can frequently help to identify strategies to make it better.
Allowing your child to try out a variety of types of toothbrushes, toothpastes, soaps, shampoos, etc., giving them a choice between taking baths and showers, explicitly teaching how to adjust water temperature and clear the shower line of cold water before stepping in, installing a flexible showerhead with a range of spray settings, at least one of which is a "soft" or "champagne" setting, and one of which is a hard jet setting, making sure that the bathroom air temperature is right, etc., can help if sensory issues are causing avoidance.
The techniques in "Smart, but Scattered", and "Late, Lost and Unprepared" can help if executive function issues are playing a role. Strategies such as having a morning and evening checklist or reminder app that includes ALL routine tasks, including specific hygiene tasks, may be useful if implemented consistently and tied to some external rewards.
I'd be sure to note and praise immediately those occasions when your child does brush teeth and bathe.
