I get past it by making sure I draw everyday whether I want to or not.
Once you entrench art as a habit the effects of feeling depressed or unhappy with the work become less pronounced. Instead of saying 'I'm too tired/sad/depressed/anxious to pick up my pencil' becomes '9:00 is pencil time'.
I suppose just following good design could cut down on those 'big variations', too. For something that seems so subjective it's amazing how taking a clinical/engineering-like stance can improve your art (not that I'm really good at that, I'm just sayin' is all).
Though once in a while I'm sure there are a few professional artists who shrug their shoulders and say 'win some lose some' when it comes to art they're unhappy with and hand it over to the client anyways. It's kinda like my piano teacher used to say. The audience doesn't know you made a mistake unless you tell them (though, admittedly that applies to little things). I suppose what I'm saying is that even though YOU'RE unhappy with it doesn't mean the client's going to be.