I've found that a lot of the images I took over a year ago are either not processed at all, or quite over-processed.
The best advice I ever had was at college when the tutor told me that I should do everything in layers, and then back things off using the opacity slider to lessen the effect. Less is more for straightforward shots.
Of course, I still can't resist the odd heavy effect, but that's more for fun.
More knowledge and experience equates to better processing. It's just an electronic darkroom after all.
As to going back and reprocessing them. Well, I tend to just re-work the original unprocessed image again. Of course with the advent of such software as Lightroom, it's easy to step backwards and undo changes that are no longer pleasing.
-H