As maura said;
Clip out the white socks (going against the hair growth) then to blend turn the clippers around and use just the tips (going with the hair growth) to think out the hair at the top of the white marking.
Once this is done, scrub the legs with shampoo (I like to use the purple/colour enhancing ones) to really get the socks coming up white. I then cover the white bits with baby powder or chalk while the legs are still wet as this makes it stick to the legs. Once I am at the venue, I brush the excess powder/chalk off and pop on some hoof black... voila uber white socks.
For face markings I just scrub clean with normal shampoo and apply a little white cover cream with a toothbrush. This allows you to coat the hair (almost like mascara) to give it a more natural look, instead of a caked on clumpy look... no clipping out needed.
Here is my Ginger Ninja, prepped as decribed above :) Capture1.jpg