Well soccerkid, let me try and help you out a bit.
-Stick with a smaller size. Maybe 400x150, 375x125, etc. Smaller it is less you have to try and fill it up.
-Try to not brush over the picture/render you use. Unless you have something specific you are trying and know a good way to get it, it almost always looks bad.
-Use colors from the render to make your background. It helps it blend in more.
-Make a border. Make a new layer, select all and go to Edit>Stroke. 1 px stroke it with a color that wont really effect the signature, black usually works till you start trying and making them match into what you already have going.
-Get new Brushes.
Here is the most popular brushes at deviantArt and
here are the newest ones released, maybe you find a good set that just came out. Changing what brushes you use, and the right brush set can really help you make a good signature. I dont recommend fractal brushes like you are using. They are difficult to get the hang of and make a signature using only them early on usually. Try Sparkle or abstract types.
-Fonts are a big thing also. Go to
DaFont and grab some fresh ones to keep your signatures not having the same feeling
-For Backgrounds, dont feel you need to fill up every space. Fill the layer with a solid color, then make a new one and start brushing. Try to keep the amount of brushing balanced throughout the signature and not too heavy.
-Tutorials. More then anything check out these for Photoshop. Can teach you a way to do something which you can then use later in your own way.
-Finally just ask. If you see something you like in someones signature, ask them how they got the effect. Some people dont like to share secrets but others dont mind, take your chances. I have asked so many people this and I learned alot. Now i just need to make my signatures look good with them
I hope these help you out a bit.