Sometimes giving the horse a bath with really warm water if you can will pull a lot of hair out - opens up the pores - lets out the hair. Also, if the horse stands in the sun - the hair sort of starts to collect and come out - I really think heat helps.
Use a round rubber curry 1st. Its great for loosening up the coat and getting the dirt up and out. I then use a knock off furminator. I then use the grooming block. Each one is great for removing the shedding coat and they also help stimulate new growth and work the coat into a nice shine. I then use a couple different brushes to remove the dirt. I use a good stiff brush first, using short brisk strokes. I then use a less stiff brush that has longer bristles and finish off with a nice soft, horse hair brush. I clean out the brushes while using them to keep them clean and work better. I would love to use the warm bath right now but our temps are still in the 40's.
R.I.P Vanny 26 yr QH Stallion 4/11/82 - 5/8/08, Scout 28 yr Paint Cross Gelding, Glistening 11 yr Arab/Saddlebred Mare