As an actor I'm constantly having to gain and lose weight. Obviously, the gaining part is easy, losing it can be the hard part.
First thing, if you drink soft drinks, then STOP! You can really lose about 5 to 10 pounds doing this alone. Plus, when you replace them with milk, or juice, your body gets so much more out of it...Many people will also tell you to cut out fast food. This is bs, you don't have to do that. If you are a student and also working, or possibly working two jobs, you're often only left with the option of picking up food quickly. Salads are an obvious choice, but if you aren't feeling a salad, get a grilled chicken sandwich without fries. This will do two things, it will keep the fried crap away from you and it will help allow portion control.
Portion control is the next thing. Way too many people eat way to much at one time. If you find yourself feeling full and sluggish after eating, then you're eating too much. Eat about 3/4 of the amount that you would normally eat, and see how that works in still giving you energy without bogging you down. And if you are able to have 6 small meals rather than three large meals, I advise trying that.
Sugar is also a big issue. Check and see how much sugar is in whatever you get. You don't want to completely cut out sugar, but you also don't want to get a can of beans that has 8 or 9 grams of sugar when you could get a can that has 2 or 3 grams. Its not a huge amount, but it pays off in the long run. Also, use Splenda rather than sugar when making sweet tea or any other kind of sweet.
They are right when they tell you to do cardio - it's the most important; however, what if you're like I am and have suffered an injury (such as a ripped tendon in your knee) and running is impossible to do without a lot of pain? Find a local pool and use it to your advantage. Bike riding and even roller blading are good as well. This will keep the pressure points from being disturbed so much. If you happen to be someone who's into the club scene, use the dancing to your advantage...just try to stay away from too much alcohol.
And as retarded as it may sound, find small habits to do while at work or wherever. If you have an elevator or escalator, take the stairs. If you're at a computer desk for 8 hours, march in place while sitting down or do stomach contractions while you're typing reports. Its a bunch of small stuff, but it all adds up over time.