Chalk! I have a grey who's constantly mucky, and when all else fails on stable stains/grass stains/legs I use chalk! Obviously it can't be used on the whole body(thats what baths are for!?), but it's really useful if your at a show and need a quick touch up! :)
Btw, sorry if I posted this twice, I didn't think it worked the first time as my computer crashed, but I think it might have done! Sorry guys!