Trying to draw things as they see them rather than using construction lines. I did this for years and now after several years of doing it wrong (thinking I knew better than everyone else) I now start at the stick figure and work my way up and will continue to do so until it all becomes second nature. Don't rush or put pressure on yourself to draw like the "masters" either. Draw the way YOU draw and don't compare yourself to others.
A big one for me was giving up too early while working on a piece. I would start drawing, and then look at it and say "this is crap". But I've learned that a lot of drawings don't look very good in the early stages, mostly due to trying to rush the process. So don't be afraid to keep working on a piece, even if you've been working on it a while and it still doesn't look good. Work on it until you either are satisfied with it, or you feel that you can't do anything else with it. Even if it still looks bad, you will have probably learned a lot in the process.
There's also the dreaded "scratchy line". I'm still trying to break that habit. :/
For me, one bad habit would be just plain not drawing enough. I sometimes go through long periods of time without touching my sketchbook and therefore my productivity and morale suffers for it, and the more days I go without drawing anything, the more reluctant it feels to draw something. One way I learned to combat this is to draw at least one thing a day, doesn't matter if it's a quick drawing or part of a project. Think of your artistic skill as a muscle: use it or lose it.