I got some more quick sketch advice in an email from Tao Nguyen, so I thought I'd share them here:
I think a couple of things will help-
1) Draw smaller. Start drawing people with about an inch or less in size. It's good practice because you don't have much room for drawing details, if you know what I mean. This way you'll fill up the paper with more drawings.
2) Try to draw in sequences like a little girl doing cartwheels, or jumping around, or a man sitting on the bench reading a newspaper, turning the pages, putting the paper down and looking around, etc. This will give you a chance to practice drawing the same thing over and over again with little variation - kinda like animating in thumb nails.
3) Draw in the air first and then when confident, put that pencil or pen down on the paper. This is the same rule for figure drawing. Sometimes I just draw a quick circle to start out for the head placement - others like to start with the body first - it really doesn't matter either way as long as you put something down. Then you can work around it.
4) my final advice would to keep practicing and don't get discouraged. Try to immitate the artists you admire. That's what I did. Just keep observing and keep drawing.
Thanks very much for those tips Tao. I'll definitely give them a go!