Have you ever been in a situation where someone says or does something incredibly funny? A situation where you know you shouldn't laugh, but you start cracking up anyway? (It happens to me all the time.) There are probably several ways to stop your laugh, but I find this one works remarkably well.
Step 1: Inhale and Exhale
First, take a deep breath and let it out again. This will help calm you down and focus on the next step.
Step 2: Relax Your Jaw
This is the hardest part. Immediately after or during the breathing stage, let all of the muscles in your jaw loosen, and bring your face to a neutral position. It takes practice, but eventually, you will be able to do this in less than a second.
Step 3: Calm Your Mind
Throughout the process, you should try you hardest not to think of whatever you've been laughing at. Instead, focus your mind on something serious or nothing at all. Different things often work better for different people, so experiment once in a while.
Step 4: Cover It Up
It is not strongly advised, but you may want to cover up your laugh and recovery by coughing, clearing your throat, or making some other sort of noise. THIS IS NOT A BEGINNER STEP. It takes much time and effort to cover a laugh easily, realistically, and effectively. If done wrong, it may give you away and lead to embarrassment or worse.