I struggle with proportions too, if you'll believe that. One thing I'm going to try out is a wire frame model layer. I drew these without it, but using a wire frame will allow for easier posing while maintaining the proper proportions.
Surprisingly, you can't really use a ruler with a pen tablet, since you can't precisely line it up with where you want the line to be. Sure, holding shift makes a straight line, but only horizontal and vertical.