Work on depth.
Creating an illusion of a more three-dimensional sig is key. :P
This is great for starters, learning to use the brushes, effects, etc but you should try and do something using a render or a stock image, and then use a stock background. This is so that you can blur the background slightly and then sharpen the image, thus creating the effect that the image is closer than the background, which it would be in real life.
Read some of the tut's here: Click Me!