Shoes produced of nubuck and suede leather will get dirty much easier than shoes made of nappa leather. Spray the surface with impregnating agent spray or protecting liquid instantly as the shoes are new and repeat this if necessary. Do the spraying outside or in an airy place. Brush the surface often with suede rubber or brush. This way you get off dirt and dust and the pile surface remains. Let the mud stains dry first and then brush them off.
You can revive the old and worn suede surface by the metal bristles of the suede brush. You may brush quite hard the idea is to open the surface again as fluffy. Some nubuck qualities even resist grinding with a fine sand paper. You may renew the worn colour with conditioner spray which contains colour.