Pretty much anything!
An excellent rule to follow is that the smoother and cleaner the surface, the better your adhesion will be. The best type of cleaner is an alcoholic based cleaner, for example, a glasses wipe or brake cleaner.
The most important decision to make is how sticky your adhesive needs to be. To learn about the different types of adhesive, and what they stick to, click here.
The next point is to ensure you apply your stickers correctly. Instructions on how to apply your sticker can be found here.