Before it gets too cold, give her a thorough bath, and use a purpling shampoo on her (quicksilver is an example) to get her coat and mane and tail very white. Don't leave it on too long, or she will really turn purple! Once she's dry (if she's good with crossties, keep her in them until she's dry- throw a cooler on her to help her dry faster and towel off her legs and face), spritz her all over with Cowboy Magic- it's pretty much the equivalent of rubbing a dryer sheet on your skirts so they don't stick. Horses tend to attract dirt, and by coating her with Cowboy Magic, you help to repel the dirt. Groom her a LOT. If she gets muddy, give her a semi-bath as soon as you can. Sometimes just rinsing off is okay, other times you may need to use shampoo. Blanket her if she's going outside for turnout- because what's she going to do outside? Roll in the mud. All horses do it. You just have to be religious about keeping her clean. Cleaning her a little bit often is a lot less daunting than doing one bath right before your show!
As for growing out her mane and tail, MTG is pretty good. It is gross, but it works great!
EDIT: And the less you mess with her mane and tail, the better they will grow. Combing it out often just breaks hairs and will give you a bunch of short hairs that stick up come show season. You can try braiding, but it can be itchy at times, and your horse may rub them out. Unless your mare gets turned out with a mane-eater, I would just let her mane be and don't mess with it.
