You've gotten awesome advice here
Ingestive behavior is more complex than we ever think it is. It might be that you're experiencing spikes in glucose or insulin which can trigger different kinds of "hunger" or "craving" sensations. You might have eaten a low-protein meal, are in the process of digesting it (even though you still feel full) and are having ghrelin spikes. Could be anything!
There can be environmental triggers which say "You should be eating" - anything from the clock on the wall to where you're sitting, or who you're talking to, even if you just ate (A big one for me is time. I tend to eat at the same time every day, and if it gets thrown off, I'm out of whack for the next 24 hours).
Anyway, water is my first line of defense, same as Pam -- and then if that doesn't work, I'll try to snack, though sometimes I'll give in and eat a big meal even when I just ate >.<
Female, 28 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 9/3/12 = 164.9 (net: -50.3 lbs.)
Current mini-goal: 160 lbs.
Next mini-goal: 150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."