I have a European toilet (one that is not filled to the brim with water) and I clean it by putting bleach in the bowl and leaving it overnight. I then put the toilet brush inside the drain pipe submerging it in the water/bleach mixture. By doing this I can use a single brush for years with it being brand new.
Also, when you flush and wipe the skidmarks from the bowl, use the flowing water to immediately clean the brush. Don't put it away with poo in the bristles.