I think that what helps me is to focus on that by saying "no" to one thing, I'm saying "yes" to some related benefit. For example: by joining SYC and spending less, I will be able to afford a trip to Disneyland with friends that will be a lot of fun. By passing up calorie-laden snacks that I overindulge in, I'll (hopefully) be able to fit back into some of my favorite clothes that are out of commission due to weight gain.
By devoting some of my after-work time to the gym rather than the internet, I've discovered that once I do cardio workouts for more than 20 minutes, the endorphins really kick in and power me through the rest of the workout, and I'm actually less hungry afterwards. And I'm already much more energetic. In late December I was really feeling low and sleep deprived, and contemplating asking my doctor for an antidepressant that had worked well for me in the past, but even after just a couple weeks of regular exercise I'm already feeling more like myself.