You really want to get a tantrum stopped before it develops a life of its own, but in real life, that doesn't always happen, so you need to be prepared with some sound practices before you have to deal with your child's tantrum in progress in the checkout line at the store.
Remain Calm: Losing your cool is sure to make it worse and don't argue with your child, either. If you can't control yourself, how can you expect your child to control themselves? Do whatever you have to do to keep yourself calm (count to 10, use the "I love my children, I love my children" mantra...) and get your own emotions under control.
Act Not React: Think before you do anything. Why is your child losing it? Is this related to your child's disability, a learned reaction, or is this a new response? Before you just react, and probably make things worse, think of what could be the real underlying cause and what you can do now.
Take Action: Intervene before things get completely out of control. Get down at the child's eye level and say, "You are starting to get revved up, slow down." This works great with the Alert Program "How does your engine run?" - Which we use and I do recommend.
Positively Distract: Redirect the child by getting him/her focused on something else. For example, you might use the line "Do you want to go look at the fish?" Some kids will fall for this one, but I know my girls are very difficult to distract once they reach the tantrum stage.
Leave: Leave everything right where it is, and calmly take your child out of the building or to a more private and quiet place until things calm down.
Hold: Holding the child who is out of control and is going to hurt himself or someone else is for safety only. Let them know that you will let go as soon as they calm down. Tell your child that everything will be all right and do what you can to help them calm down.
Time Out: Time out works effectively if you apply it consistently. My kids know that there are corners (our naughty spot) anywhere we go. If you are embarrassed, take your child out of a public place, use your car if you need. Come down to the child's level and calmly explain why they are in time out and how long they will stay there. Use the one minute for each year rule.
Wait It Out: It is an exercise in futility to attempt reasoning with a child - especially one in the middle of a total meltdown! Often, the tantrum will just have to run its course, so depending on where you are you may need to place the child in a safe location to wait it out. One of my daughters (she is 10 now, so this has been 5 or 6 years ago) could throw a fit the likes of which you would not believe. Dealing with tantrums with her was literally impossible, so we would put her someplace quiet to ride out the storm. Sometimes it took several hours before she wore herself out and fell asleep.
Do Not Be Tempted
Never give in to your child's tantrum. Oh yes, it is sooo tempting - especially in a public place to just give in, but don't do it! The hardest behavior to change is behavior that has been intermittently reinforced - which means if you give in once or twice just to keep the peace in the grocery store you have taught your child that if they just keep up the tantrum long enough or loud enough they WILL get what they want. So they continue to tantrum at every opportunity - you know - like every time you have to say "no" to something your child wants. Just don't give in! (No one said this was going to easy.)
After the Tantrum Stops
The post tantrums cool down stage. This is the time to talk and reinforce the lessons you are teaching your child about behavior and communication. Listen to your child and try to get to the real source of their anger or frustration. Try to solve the problem together, as this is a good time to get your child to think and come up with solutions and alternatives on their own. Use the "what do you think would be a better way to..." approach to encourage their input. This is also a good time to teach new techniques and ways of dealing with emotions, recognizing the signs of losing control, and ways your child can take action on their own to stay at that "just right" state.
Love Them: Make sure they know that you love them and approve of them; it is the behavior that you will not accept. Talk about alternative ways of getting what they want or acting in that particular situation.