When you're stuck for ideas, or just warming up for your painting session, try knocking out some small, fast studies (try for 20 minutes).   Remember how it was when you were a kid and you just invented things to draw?   You can still do that, and practice painting at the same time.   It's surprising what emerges sometimes.  This first little figure has a plaintiveness that I really like.

Think about one or two elements while you're doing these--proportion/drawing, value grouping, color mixing.  The benefit is in "thinking with your brush" rather than trying to rationalize.  Every bit of practice establishing correct figure proportions will help when the time comes to work from a model.

If you don't have a model or any suitable photos, grab a fashion catalog.  Put up the photo and look at it as if it were a model, not right beside your canvas.  Try to get the gesture and proportion.

It's not essential to know human anatomy when mapping out a figure's proportions, but it is SO helpful if you want catch a realistic gesture.  Arms and legs are not just cylinders, and the shapes are in specific places for a reason.  I did not enjoy having to draw the skeleton and muscles over and over in art school, but it makes a world of difference to know how the muscles overlap and what structures underlie the shapes that you see.  In fact, knowing these things will enable you to see shapes that you would otherwise miss on the model, and to portray them convincingly even if your reference material doesn't show much detail (like this figure above).  If you're going to do much figurative work at all, I strongly recommend getting some good reference material on human anatomy and physiology.  This one is my all-time favorite, but there are many inexpensive guides (like $4 or less) also available which will get you started. 

And remember, if you paint from a copyrighted photo which is not your own, it should be attributed if made public.  These last two studies are based on images from a Hanes catalog.