I havent written a tutorial in ages! Plus Bjenks was wondering about multi layered sigs, so here it is.
- Open your favorite sized document
- Create 10 layers, you should have 11 layers total.
- Fill layer 1 with white.
- Pull out one of your grunge/abstract brushes and lightly paint with black on layer 2.
- Do the same on layer 3 with the same brush but white.
- On layer 4 use another brush and paint with black.
- Same brush on layer 5 with white.
- Add a border and your done!
Or,
... Continue for multicoloring ...
- Put layers 6, 7 and 8 on soft light
- On layer 6 use the first brush you used with a color of your choice, try not too use overly bright colors.
- Same brush on layer 7 with another color.
- Use the second brush you used on layer 8 with another color.
- Select the polygonal lasso tool and put the feather on 20.
- On layer 9 make something like this with the lasso and paint bucket.
- Do the same in layer 10 and fill it in with another color
- Again with layer 11.
- Hold control and click layers 9, 10, and 11. Release and press Ctrl+E.
- Put this layer on soft light or overlay.
- Add a border and your done! Congratulations!
Sorry about the links, Ima have to talk to taco about that.
Cyall around!
More Tutorials: 
