You can't really have a waterloo game with out taking the mud into account. I used this muddy technique while painting up my Roman baggage train and liked it, so I thought I would use it on the French field guns.
The mud was very easy to achieve, I simply painted PVA glue onto the wheels and flocked them. I then painted them brown before the glue had time to dry this way the brown paint and glue mix while still wet. As PVA glue drys shiny and wet looking, it makes ideal fresh mud.