I may have written about this before, not sure, but: Managing contrast in a painting -- representing/getting down dark darks and light lights -- goes a long way. They (very dark and very light tones) are tricky because they can contaminate, or get contaminated by, other tones, and because they can dominate your image -- they can be too "loud." But they need to be struggled with -- if you don't get them in there, you end up with an all-middle-tone, muddy, lackluster painting, rather than something that has an impact.
I have a tendency to get lost in color and not remind myself to attend to tone; I need to keep a tighter grip on whether or not my tones are accurate. And various enough. They need to range across the spectrum (of tone -- light to dark) just as my colors need to range across the color spectrum.
Apart from thinking about contrast, I'm trying harder than ever to paint "freely" -- to dive right in and get things down quickly and without fuss, in whatever way my hand moves, whatever way I see things the first time, whatever way it "happens." (I have been invoking Wolf Kahn's phrase, "Just let it happen.") And I am trying not to second-guess those initial marks too much -- just adding any obviously needed brighter or darker tones, or more intense color. Corot's comment that "Everything that was done correctly on the first attempt was more true, and the forms more beautiful" has been ringing in my ears: I see that happening too, and I want to let that "trueness" stand, not beat it down or stamp it out with second-guessing.
Also, I have been musing about something I heard a chef say in an interview with Charlie Rose: that each day he tries to do better than before, to improve. That is why, as an artist, you have to keep pushing past what you've done before -- never trying to replicate it, but seeking a way to move forward. Make something new, that you like even more.
So you can't think too much about past work, past paintings. You can recognize in them things you like, and remind yourself of the frame of mind you were in when you created them (how much you cared/took care vs. how much you let yourself go; how much of a hurry you were in; how much you allowed your imagination go wild vs. reined yourself in), but you can't try to do the same thing again. To repeat yourself is like trying to apply a formula, and formulas don't work in painting. Or rather, they don't work to open you up to risk and exploration; they work only to constrain you. And it's letting go of constraint that seems to work best -- to create exuberant, evocative work.