What I've always found helpful is to pick a line, or set of changes, or whatever, that I know COLD. I'll get my handy metronome out (I heartily recommend grooving with a metronome whenever possible), and with the click on the 2 and 4 (or 1 and 3, or just the 1, whatever suits you and gives you 'space' to stretch in) and play the line for a few minutes like I know it. Then I'll try to play it a smidge quieter, once or twice. Then a smidge quieter again. And again. This will rinse and repeat until I'm barely audible through my amp (I'm talking 'lightly grazing the strings with the very tippy-tops of my fingertips' here), and then I'll go back to playing the line 'as I know it'. Once I'm acclimated back to the 'base', I'll play the line a smidge more aggressively. Then again... you see where this is going. I stop when I'm rocking out like Animal.
The trick is to find your own definition of 'smidge', as a unit it's pretty subjective. Then when you're onstage, you'll have a MUCH greater awareness of what level of smidge-ness you're at, where the drummer/rest-of-the-band is at, and you can calibrate yourself accordingly.
Another fun one that's a bit more loose than the smidgeometer is to take a line/song/whatever that you know cold, and play it "differently" (again with your trusty metronome). Play Rush anthems like ballads. Play ballads like head-bangers. Play the changes in 3/4. Play Slayer like you're trying to woo your girlfriend, etc. The point is to break out of the 'viewpoint' the song is already recorded in, and is also very helpful at developing your dynamic awareness.
Anyway, hope this helps!