Number One - Buying too much on credit.
No matter what income level, more people get into financial trouble because of too much debt than any other reason. "Too much" means different things to different people. Very few people go through life without making a purchase on credit. However, trying to "keep up with the Joneses" rushes too many of us into lifestyles we simply can't afford. Buy some things you need on credit, like a home or a car, but save up the cash to buy the things you want.