This is pretty simple. Firstly, try not to get your Nubuck and Suede shoes too mucky. If they’re clean, simply brush then as regularly as you can with a stiff brush designed for Suede and Nubuck. This raises any bits of the “nap” that have been flattened and gets any loose dirt off. Brush any dirt off the soles as well with a nail brush or something similar.
If they are dirty – buy a proprietary Suede/Nubuck cleaner like the one on the left and follow the instructions.