One of the best tips I can give is windex. Along with razor blades I think I use it in almost every project. You can take a corner shelf that was left outside in someones yard, which they gave to you for free, and with windex, elbow grease, a little sanding and some paint it can be better than new. Windex will remove dirt, grease and oil build up its a great first step in prepping a project.
Before and after with just using windex and a lot of paper towels. After this was complete I put a few coats of yellow on it and sold it for $20 at a craft fair.