Reverting to your old ways will bring you right back to the previous number on the scale. That's because your new, smaller body requires fewer calories to sustain basic functions (like heart rate and breathing) than your larger self did.
The fix: Give yourself a new calorie limit. You need eight fewer calories per day for every pound you lose. So if you used to eat 2,000 calories and you drop 20 pounds, you'll need 160 fewer calories now―1,840 total―to keep your slimmer profile.