I do not suggest tutorials, instead I suggest you keep trying new things until you find something you like. I definitely see improvement and potential in all of your sigs, you just need to try some more new things. Stay away from brushes, maybe try some smudging and C4D. Stay away from text as well unless it's a watermark and really try to focus on using stocks and renders rather than renders and brush-like backgrounds. You've got a lot of potential, and I'm sure you can get much better in no time.
I agree with P h o e n i x, but I would like to add that you could (I recommend) search some tutorials about blending, depth and flow, those are three of the most important.. 'parts' of a tag, and not only a tag, of every work that is digitally done. One method I really like, and you could also try, is photo-manipulation, basically, 'manipulating' a stock image, adding depth, flow, and better blending it etc. Really great for improving these three skills. Anyway, I like your forth tag (I'm a big DBZ fan). The text rocks there ;P Keep practicing, and remember that practice makes perfect (or so it's said. ;P)
yeah, unless you know nothing of photoshop do tuts, otherwise keep your style. your sigs are good, they are straight to the point, i like that.
