My biggest mistake when I first started was trying to get more distance out of my clubs. When I stopped that and focused on getting on the fairway (no matter how short on the fairway) my scores began to lower. That in turn seems to have helped me focus more on a smooth well timed well controlled swing which has created to my surprise much more distance with my irons than I ever had trying to beat the ball into submission.
I am only just now realizing the benefit of good instruction but with little time and less money I have decided to try to just play with better players from where I work and be open to their advice.
Make sure your clubs fit you, that alone will keep you from developing poor swing habits trying to make your swing conform to poorly fitted clubs.. I had a guy I was playing with say one time. "man you are so hunched over when you hit. I then looked back and could see myself with a bad posture because my clubs are too short for me and when I would get a good connection the ball would go straight right because I could not keep the sole level with the ground, or I would top it because I was trying to correct my posture. Needless to say I will not break a 10 handicap until my clubs are lengthened.
Hope that helps