It's easy to operate on autopilot ― repeatedly going from hand to mouth ― when you munch straight from the package. With this kind of mindless noshing, you can quickly lose track of how much you've swallowed.
The fix: Measure out a portion. Put a single serving (check the nutrition info!) on a plate or a bowl and sit at a table, not on the couch in front of the TV. This will help you stay aware that you are actually taking in calories.