First off, let me start by saying this list is by no means exhaustive. However, if you can manage these 4 important components of a weight loss program, you will be on your way to a slimmer body.
Build the Foundation
Many people begin a weight loss program with their basal metabolic rate (BMR), which is the amount of energy (calories) your body uses in a resting state, in the gutter. The key then is to increase your BMR so that you are burning more calories throughout the day, every day. Increase the amount of calories you burn, increase the amount of fat you lose. Makes sense, yes? The best way to increase your BMR is through adding muscle tissue. Females often shudder at the idea of adding muscle due to their fear of becoming “bulky.” Relax babe! It's a little harder to add that kind of muscle mass than you think. Besides, a little muscle mass goes a long way toward achieving a leaner body. And this is what you want, right?
Specificity of Program
Not just any exercise program will do for weight loss. Simply burning calories is not good enough. The program must be designed to elicit specific hormonal responses that are conducive to weight loss. I'll give you a hint: hours and hours of cardio is NOT the answer!
This one pretty much speaks for itself. Have you ever achieved anything great with an inconsistent effort? Unless you consider failure a great achievement, you probably haven't. You must exercise consistently to get the results you want! It amazes me how many people put so little effort into their exercise program and complain about not seeing results. Even the most well designed programs are useless if they are not adhered to on a consistent basis.
Again, this one goes without saying. You can't expect to achieve a leaner body by continuing your unhealthy eating habits. This isn't to say that you should drop everything in your diet immediately and exchange it for “healthy” food. You would end up miserable and revert back to your old habits within a month. Instead, practice moderation and don't overindulge. Also, phase certain unhealthy foods out of your diet while introducing healthier foods. The transition will be easier to make and a lot easier to maintain.