--Branch out to other genres. Recording/Mixing heavier music is such a singular skill set, and certain aspects of it don't translate to other styles. Try and mix a country or folk record, when the options of impulses/reamping/samples/clipping etc etc aren't really in any way possible. The more styles you mix in the better you'll get at all of them.
--Separate the creative and technical sides of mixing. For me that means doing all the bullshit editing/pre processing and samples at least a day before actually mixing. Sometimes I even mix in 2 stages, where I would start with general tone shaping, then spend a whole other slot of time later just doing automation and fx. I don't know about most of you, but I find it impossible to get inspired when I spent an hour picking samples, and fixing shitty vocals.
--Tracking wise, get the sound at the source, but when in doubt, go neutral. Nothing worse then an overly bright/dark guitar tone. I'd rather get a boring, but clear set overheads that I can manipulate, over an eq'd to hell and back bright set.