I have written numerous things on the net about coping with meltdowns. With me it's the fight or flight reaction to stress... If I can't fight, then I flee (possibly take a blind run). If I choose to fight, it could be screaming, tantruming (think 2 year old style), and self injurious or abusive behavior.
Over the years, my meltdowns have reduced in frequencies. Some of my coping mechanisms are to recognize my feelings. What is it that is upsetting me? How do I need to work it out? Chances are that thought will be on my mind and that stressor will be snowballing...
No matter how "minor" that trigger thought is DON'T try to rationalize myself out of that feeling... Often the worst meltdowns occur not because I am upset over something, but because I can't control the feeling I have about being upset over something. Rather accept the fact I can't control how I am feeling at the moment. Rather than trying to stop feeling upset, I'm best to just let go and try to shift my thinking to something else unrelated to the trigger, and keep doing that each time my mind wants to pull me back to the problem not resolved.
Eventually when I feel better I can try to return to the situation. If it makes me cry again, then time to repeat the process until I can attack from a better frame of mind.
If the stressor is an argument (seems I have more meltdowns at home than anywhere else), then I need to get the heck out of the situation the moment I feel like I'm in a losing battle. If the person continues to persist, I need to tell them to stop. If they don't stop, I need to leave again. If they don't then I may just lose it...
If the stressor is environmental, is it something I can change? If I can change it or resolve the conflict then try to do so. If it's something beyond my control, then I need to find something to distract myself. Also it helps sometimes if I can gain full understanding why the change is necessary. I may or may not agree with it, but if I can see the logic behind it, makes it much easier for me to cope. One of my frustrations is dealing with confusion. I don’t take confusion well at all. I _must_ understand the world around me… If you’re going to change my world tell me why.
If I'm stressed out I need to escape the situation and find something else to occupy my mind. Same thing if the stress is environmental. I need to get out of the situation. If I have to wait in a noisy building, I try to see if there is a chance I can wait in an abandoned office. If not then I try to find a distant chair... One of my frustrations is dealing with screaming children under 5... The squealing and screeching hurts my ears.
In stores: try to go at less busy times. Find a quiet aisle I can retreat to if I feel overwhelmed. Also try to get what I need and not spend too long.
Also pay attention to what my body is telling me. If my feet hurt, don't try to go on like I feel nothing. I think that's one of the misleading things about autism; that people feel no pain. I'm sure they feel it like I did, but tend to ignore or tune it out. There comes a point it can't be tuned out. Once that point is reached, it's too late. That may explain some of the unexpected meltdowns with many people...
These are some things that help me. Hopefully they can help other people too.
And here are a couple articles with more information on meltdowns I found interesting and worth reading:
I hope you find them useful.