As Elian said never use a Bevel or Emboss effect, try to refrain from using strokes, and don't use 50 different colors for one piece. Study the color spectrum (or just look at the one they supply you with) and see which colors contrast with each other. (Colors that are across from each other on the Wheel), for instance Blue and Orange contrast really nicely.
A lot of people will bring up the "Not GunZ Colors" just ignore that because saying that a GunZ Interface has to be a certain "GunZ Color" is foolish. Make the interface detailed, instead of just using a Rounded Rectangle for a Border/Outline, make it a little thicker, create a new layer with an image on it (any image), select everything but the Border/Outline on the Original layer, click the new layer, delete, and mess around with some layer properties.
Add little things in here and there as well, or maybe alter the Tables a little bit. Most important; Clean, Smooth, Colors Flow, and Simple. (Sometimes Simple may not be the best option though) As I said though don't have 50 different colors flowing around everywhere, stick to 2, maybe 3 Colors that all go together nicely. Or even set a Theme you want to go by, Metallic Theme, Ocean Theme, Fire Theme, etc. Like segaomega's monochrome interface.
