Step 1 = Keep your toothbrush in mouthwash.
Where does your trusty toothbrush live?
If it is residing on the side of the sink, in a pool of handwashing water (which means that it stays damp and bacteria-friendly) then you really do need a toothbrush holder.
If it is already in a toothbrush holder, is it near the toilet? I remember my horror upon learning that particles of faeces and urine can fly 2m up and away from a toilet flushed with the lid open! Urrgh! This may seem a tricky problem to solve, as who has a bathroom large enough to have a 2m exclusion zone around the toilet!?
One solution is to train everyone in the house to 'SHUT BEFORE YOU FLUSH' - this will be an ongoing battle as it is hard enough to get others to put even the seat back down after weeing!
Another solution is from an article in Good Housekeeping (sometime in the early 2000s) - to keep your toothbrush head-down in a see-through plastic toothbrush mug, with enough mouthwash in to cover the tooth brush head completely.
My mouthwash of choice is a plastic-packaging-free, tablet option. Georganics Spearmint Mouthwash Tablets come in a glass jar with a metal lid (so no plastic packaging to send to landfill). There are 180 tablets in a jar, so although the £8 price tag seems high, this mouthwash lasts for ages and ages (over a year in fact!). I put one tablet, in 1cm of water, in my tootbrush holder (after cleaning it out), every few days.
Toothbrush holder mugs often have an un-noticed slimy layer of communal saliva and whiffy bacteria in their base - does yours? This concoction is spread over the hands of anyone who keeps their toothbrush there. No wonder colds and bugs spread fast through a household!
The toothbrush holder really needs to be see-through so that you will notice if the mouthwash in the mug is looking manky and unattended. (It needs emptying and scrubbing every few day, before rinsing and topping up with mouthwash).
I always choose a plastic toothbrush holder mug, as it's easy to knock over stored toothbrushes (causing possible damage to persons or ceramic sanitary-ware if glass or ceramic falls).
In our bathroom we have the IKEA Svartsjon mug (£1.90) with the cheapest value supermarket mouthwash in it. On a safety note, we always rinse the toothbrushes in our household in tap water before and after use - and the mug containing the mouthwash is kept high up out of child-reach of course.