Understand that laughter is good. If in a comedy, it means that the material is fresh and funny enough to cause an actor to break character.
Take deep breaths. Sometimes a few deep breaths will cause the laughter to gradually lessen in volume.
Focus on your lines in the scene and say them over and over in your head, with your eyes closed.
Turn away from the cause of the laughter and take another deep breath to regroup. Turn back around and repeat Step 3.
As morbid as it sounds, force yourself to remember something incredibly sad, like the death of a pet or family member. This sounds odd, but sadness eradicates laughter.
Bite your tongue. This is a trick many in the film world employ in order to stop laughing. For some reason, the nerves of the tongue are sensitive enough to stop laughter right away.
If none of these work, simply take some time off. Ask the director to CUT and then take a half hour to regroup. This is better than ruining multiple takes and wasting film.