As far as keeping a painting fresh to the end, you can not lose sight of the reason for starting the painting in the first place, that first excitement, that one big relationship, if the details slowly obscure the big thing the painting becomes dull, then it is necessary to dig back in and pull it out even if it means upturning days of work, in the end nothing is lost and it will be more exciting for being harder found and deeper felt.
Try not to dilute the paint. (There is a time for thick and a time for thin.)
Instead I am always mixing on the palette and on the painting; going up and down the value scale, from light to dark, from dark to light. Do a painting. If it works out well, that’s great, if it doesn’t that’s great. You have the perfect surface for another painting and it solves the problem of starting with a blank sheet.