If you started gaining weight recently, maybe something in your routine has changed. You could try to figure that out. Sometimes stress can cause weight gain too.
Otherwise, if you don't already exercise consistently, you could find a form of exercise that you enjoy and do it consistently (or more often).
If you're a fast eater, try slowing down - sometimes you are full but your stomach hasn't told your brain yet :D
Hope that helps and good luck!
Lol. I am very lazy to exercise and eat erratic ..