I think I mentioned in a previous post that I used to think that a drawing that couldn't be done in one sitting wasn't worth doing. I also feared that my technique would be noticably different from one sitting to the next. Now my drawings take several days, not only because they take 12 hours or more, but also because a stay-at-home mother of a one-year-old can only have so much time to herself. I can see now how wrong I was.
First off, once you have a style or technique, it doesn't change much from day to day. At worst, my mood affects how hard I press the pencil and how fast I go, but it's really not noticeable in different areas of a drawing. And if I feel particularly out of sorts, the best thing is not to draw at all.
But the biggest thing I noticed is how much a little time away can help. It's more than just stepping back and looking at the drawing from a different angle. With detailed drawings, I get so intense that I sometimes don't see the "big picture." By putting it away for the night, I notice little things that could be changed when I return. This area could be darker, that needs to look fluffier, this line isn't right, whatever. Sometimes it takes fresh eyes to notice certain things. So take a step back, put it out of your mind for a while, and sleep on it. When you return, you'll see things you didn't while you were working.