Well I kinda agree with 25000st point there. The thing actually is that you don't usually see many great tutorials that explain anything beyond beginner-intermediate level. Most of the effects we see in them are confined to that particular sig itself. And usually you already know what to do with them. And I am sure you all agree that finding tutorials for lets say.. 'EPIC' sigs is a bit hard. Whereas most psd/xcf packs you find at deviantart are AMAZING. To me they help me more than any tutorials. Primarily because when I look at the stuff inside a good psd, it clears a lot of my concepts about depth, composition etc. They give you the ability to play with the layers and effects inside which also in return greatly help you.
The only tutorials I ever found that helped me learn something useful about signature making were...http://senthrax.deviantart.com/art/Full ... l-55042010http://zenron.deviantart.com/art/Zenron ... -164141102
(this one is basically talking about almost the same thing as the above one, but some extra details on those matters don't hurt.)
Even if they are Photoshop based, they deal with almost all the basic concepts that you need to learn the art. A must read.