If you are going to wash, really wash.
If you will be washing your horse before clipping, it is a good idea to make sure you get as much scruff out of the coat as you can. Firstly, use warm water rather than cold if possible. As well as feeling nicer to the horse, warm water will help shampoos to lather up better and as a result will remove more scruff and grease from the coat. To really clean a coat, mix warm water with a cleansing shampoo (fairy liquid, wahl dirty beastie are both effective as is hibiscrub which is not a shampoo as such but will really clean a coat) and a capful of Dettol. Lather into the coat and rinse thoroughly. To help dry your horse faster after washing, mix some methylated spirits into the rinsing water, it will help evaporate the water from the coat faster.