I was (and often still am) in the same position. What started working for me was to draw the same pose over and over, constantly correcting the proportion errors. Once I got to the point where I could do a decent gesture three times in a row, only then would I move on to the next pose. My personal tendency is to make people too wide and stocky, so after making the same mistake over and over, I started "seeing" how to make my gestures more narrow and graceful. What also worked for me was to ignore the clock. When I would attempt a 30-second pose, the only thing I accomplished was to create a terrible drawing very quickly. Now I take my time and really think about where the lines need to be. My 30-second poses take five minutes. But after I've done the same pose a dozen times I find myself getting faster and faster.