I never had anything on my DS for the first 2 years cause I liked the sleek untouched look, but I've recentely put a little sticker of Mario and Yoshi from SSBB on the top and 2 pokemon charms (turtwig and pikachu) where the strap goes, just for something different.
My girlfriend bought one of those DS sticker packs that are character themed with litter stickers and little silver stars, far to pretty for my liking but it doesn't look to bad.