Calorie counting.
I eat 1500 or fewer calories per day. I have done so since October 19, 2010. The only exceptions were planned--Thanksgiving and Christmas--and I "paid back" those calories by eating 1400 calories a few days over the following couple of weeks. I have been 100% on plan the whole time--and yes, I count the holidays as "on plan," as I planned for and enjoyed them.
It's the only thing I've ever done to lose weight that was A) cheap, B) effective, and C) sustainable. Of course, I had to make peace with the fact that I'm losing a pound a week or thereabouts and not the two pounds a week I lost on Jenny Craig or the five pounds a week I lost for the three weeks I tried Atkins.
The flip side of that is that I spent a ton of my parents' money on JC when I was in college (thank you, Mom and Dad), and I was only able to do Atkins for three weeks before my brain snapped and I upended a barrel of Chee-tos over my face.
What good is a plan that I can't adhere to? What good is a plan I can't afford? I've finally learned that the answer to both those questions is "no good at all for me." I'll take my 6.3 pounds per month and enjoy the fact that I have been able to actually adhere to the diet for six months. It's WAY better than flitting from plan to plan in desperation hoping for the Quick Fix that never comes.
I exercise as well, but I consider that separate from my weight loss. I do it to elevate my mood and improve my health; any weight loss I get from it is a happy side effect.