Hey,
What Yo said makes good sense.
Also try just warming up before sketching by drawing circles, lines all different sizes. Look where u want ur pen to go and not at the end of ur pen.
Use different pens, markers, pencils, highlighters, tablets etc. Even if you find a media which suits ur style continue to try new media and go back to ones uve tried, this will continue to develope ur induvidual drawing style while not letting it become too accustomed to one media.
Dont go back and fix up ur mistakes. Its not technical drawing. Drawings always look better when their looser. If you fix one mistake, you have to fix them all.
The best advice would be to keep drawing. The people who can sketch the best are always the people who constantly draw and doodle and have thoursands of hours sketching time to their name.
Check out http://www.cardesignnews.com/studio/tutorials/index.php for a few helpful sketching tutorials. Also remember ur ur own worst critc so ur never as bad as u think u are.
Good luck! Yeah and we want c some pix