Off the top of my head these are the first to come to mind..
1. Use a lifeline especially when doing lace, but it's handy for any large project you don't want to have to frog back to the beginning.
2. Learn what the stitches look like. I can't emphasize this enough! Once you learn it's so much easier to 'read' your knitting and know where you are in a pattern or see mistakes.