A combination of lotions and potions! but would also swear by the sportbac boots, particularly if your fields get really muddy.
Sudocream is always a favourite or udder cream.
Also found it useful to get the 'leg wraps/warmers' for the stables as it keeps there legs warm and dry especially if you dont have masses of time to ensure they are really dry
An irish company called Foran have a product called M.R.S cream, was used on the TB stud I used to work with, after about 3 or 4 winters the mares built up resistance but it was fantastic, great barrier and also great for actually clearing up scabs and sore areas. http://www.forans.com/products.asp?Action=Detail&ProductID=101
Link to the page.
Athelets foot powder, my stallion has had it bad on all four heels over the past week, on one foot it was very bad, its cleared it up in days, i always apply it when hes kept in, to allow it work, then if hes going back out use sudo cream as a barrier.