How do i keep weight off after losing it?

i lost about 7 to 8 pounds. i would eat a bowl of cereal for breakfast, something like special k protein plus. i wouldn't each lunch most of the time. then i would have soccer practice or a soccer game after school. (the only reason i didn't eat lunch is because i play goalie more then half the time and very rarely do i play on the field as a mid-fielder. so by playing goalie i get very little exercise.) anyways....i did this for about 2 weeks but the first time i ate a meal until i was full, i gained back about 4 to 5 lbs. how can i prevent this from happening again?