descending on a fixed is definitely the hardest part. brake pads are cheaper and easier to replace than knees or faces. if you have brakes, use 'em.
you'll eventually get used to always pedalling, but you still might forget at times. i've been riding a fixie for over 5 years now and i still have moments of forgetfullness... but i ride lots of freewheel bikes too... which i sometimes forget can coast.