1. Remember, the rewards, while great, are not worth the debt. Many people will spend on a credit card over a debit or using cash because they get reward points. While there is something great about flights, electronics, gift cards, and even cash, how much are those things actually costing you in interest? If you choose a reward credit card for the rewards, and overspend in your quest to earn a round trip ticket, or a new iPod docking station, you might pay much more in the long run than the cost for those items would have been flat out. Do not let rewards suck you in. If there is something you want, then buy it, not using credit, and not using rewards. Instead of saving up reward points, save up real money, and get it without paying significantly more (in interest), then you need to.
2. Low introductory rates are the trick and the trap. Credit card companies offer no or low interest rates for the first six months, or year of the card. This means you get in the habit of overspending, and justifying it because you aren't paying interest. Well, when those 6 months of interest free glory are up, you are stuck paying an enormous interest for items you probably won't even remember buying, like lunch at a fast food joint, or a new t-shirt because it could be cute. No matter how great the introductory period is, don't get trapped into a card unless you find that the rates after the introductory period are worth your while. No matter how certain you are you can have it paid off in time, it may not happen, so don't bank on it.
3. Calculate real cost. Whenever you are tempted to spend on your credit card, take a moment to calculate the true cost of the item, assume the worst about yourself for a minute. Say it takes you a year to pay off your balance. So, if you buy a two dollar gallon of milk, and pay 20% interest on it for a year, you are paying closer to seven dollars for the gallon of milk. If it was sitting on the store shelf with a seven dollar price tag, would you pay that much for an average gallon of milk, nothing special done to the cows, etc? No, so recognize that you would not pay it if you knew the true cost, and figure that out before you whip out the plastic. Knowing what it could potentially run you might sway you from overspending.