sig tuts are rather useless. because sigs are rather the easiest crations of photoshop... -_- though some sigs are rather complicate, but most are simple...
the main thing about sigs are blending, like for example you have an render, find the base colour of the render, and then use that base colour on the background...
for backgrounds, you could either use and normal image, or and editted iamge... or painted it with abstract... normaly to paint abstract is easy, just get an abstract brush... but never extend the brush to a really high setting, or the abstract would be very pixelated... which is horrible.
text, don't always add too much ebouse and something else, try to just like leave it as the way it is, maybe add a bit of effects. don't add too, much, or it would be HORRIBLE! since too much effects, or too big of an text, can make the sig unconsiderably ugly...
http://search.deviantart.com/?search=photoshop%20brushes%20boost:popular%20age_sigma:24h%20age_scale:5+in:resources/applications/psbrushes§ion=browse
average phooshop brushes
http://search.deviantart.com/?search=abstract%20brushes%20boost:popular%20age_sigma:24h%20age_scale:5+in:resources/applications/psbrushes§ion=browse
abstract brushes.
download them with the download button of the side. save them into
program files>adobe>photoshop(whatever version you have)>presets>brushes
to add them in, go to the brush palette, on the top if ur using CS2, and then go to the little arrow thing on the side of the brush button, then go down to load brushes, or reload brushes. and then go to
program files>adobe>photoshop(whatever version you have)>presets>brushes and then cklick your downloaded brushe, then okay.
though i recommand reload brushes, if load brushes, then the brushe palette gest awfully messy and unorginized... and you can always reload your other brushes...
Best of Luck
EDIT: try not make such of an big sig...