I had problems with doing sketches also. A few things helped me were doing blind contour drawings (you do just one line to represent something, and you don't look at the page) and using very small pieces of paper and setting a time limit. One way to enforce the time limit is to move yourself after a set amount of time (10 minutes?) so you CAN'T keep doing the same drawing.

I'm using an icon here from my favorite erotic contour drawing artist, Francois Dubeau.