No matter how convenient it might be to use your credit card, do not use it to pay for anything if you can use other methods of payment. If at all possible, pay cash. Paying cash will give you a firmer grasp on your financial reality. You just held the money in your hand. You've seen how much will be left in your wallet if you spend this money now. You can feel immediately that you are parting with a certain amount of money, which is very different from paying with that magical plastic card. The "play money" you use to buy all those things you really do not need.
References:
http://www.debtorsanonymous.org/
http://soundmoneytips.com
http://credit.about.com
http://www.happiness-project.com/happiness_project/2009/02/money-9-tips-to-avoid-overspending.html
http://www.ehow.com/how_2099112_stop-overspending.html
http://abcnews.go.com/GMA/Business/story?id=1521979
http://ezinearticles.com/?Decreasing-Your-Credit-Card-Limit&id=164271