Don't go to the mall
Pretty simple...avoid places you're most likely to spend money you don't have.

Don't have a credit card
Online purchases especially...things you can't afford.

Don't spend what you don't have
Don't buy an LCD TV if your "old school" one hasn't shit the bed yet. Don't buy another pair of shoes if you already have seven pairs in near-perfect condition.

Stop eating out/Learn to cook
Dinner is way less expensive when you buy the ingredients and make it yourself. 100% of the time.

Only go places you NEED to go
Gas is still wicked expensive, especially for those with tighter budgets. Only go to work and/or school, take a trip to the grocery store every two weeks instead, meet someone halfway instead of trucking all the way to their place.

Hell, Budget!
Plan what to spend money on; set aside specific amounts for food, bills, and emergency cash (if your car breaks down, if gas prices go up, etc.)