My favorite points from Mike Rundle's excellent post on user interface design include:
Keep it crisp. No blurry lines or edges.
Always adjust opacity. Nothing is totally black or white, dark or bright. A semi-transparent black or white line, glow, shadow or shape goes a long way."
Real-world objects rarely have perfectly square corners. Use subtle rounded corners to make objects look real.
In real life, everything casts a shadow. Unless you're drawing vampires, if you intend your object to have depth and be resting on a surface then it better have a drop shadow, even an incredibly subtle one.
