What brushes do people use when they make the background layer for popout signatures, I can never find one that looks good. Everyone else has like the same style that decays at the edges but looks crisp and i can never find a brush that gives that effect

You could probably just use any "abstract" or "grunge" brush set really. Just look for a few with decently defined edges instead of feathered.

http://browse.deviantart.com/art/Metal-CX-s-Chaotic-Abstract-19548105

Would probably work fine for what you need. If you're using any kind of drawing tablet just swap over to a mouse @ 100% so you can click once and get a solid shape. You might have to look at the brush settings of a brush in that pack and turn some stuff off to control the outcome a bit more.

ty! i was also looking for that kind of brushes and they look pretty nice!