Every Aspie I imagine deals with stress differently. I've learned that sometimes prevention is just as important than knowing how to calm down.
Being highly attuned to our senses, it's good to recognise what sets off our stress, and what makes us happy. I myself try to compliment as many senses as possible, for maximum comfort, which is more likely to prevent a meltdown in the first place. Of course you can't have these creature comforts 24/7, and meltdowns will still happen, but it can greatly reduce them.
Here's some examples for myself. You can try some of these if you want, or just test different things out, and maybe start a 'meltdown' list; maybe even have a 'cool down' room, or space, for you to relax. Do these things daily, weekly, whatever, to keep the stress down:
Taste
Comfort food can help a great deal. Be careful of course not to indulge too much, as to affect your health in a bad way. I like to have roast meats, mash, salmon, Oreos, or make a chocolate mousse (I have a healthy version recipe if you want).
Smell
Find out what smells comfort you, and make them accessible. You can use candles, incense, perfumes, soaps, whatever works for you. My favourite comfort smells include vanilla (of course), oranges, rock melon/ cantaloupe, and lemon.
Sound
Sounds depend on how we feel; compliment your current mood, and try to avoid music that is sad or angry, unless it's all you can stand to listen to at the time. I like soundtrack music, as it allows you to clear your mind, rather than concentrating on lyrics. I have a few suggestions for soundtrack music if you're interested. I also like the sound of rain:
www.rainymood.com. Sometimes silence really is golden though, so if all else fails, try that.
Touch
Generally soft fabrics are my go to here. Bed sheets, cosy blankets, pillows. If you like water, maybe a long shower/ bath/ swim, or if you have a pet, try simply petting it; it's very therapeutic.
Sight
Being surrounded by beauty can soften our hearts. I like to collect art works online, from various sources, and simply place them all in one folder, to peruse when I'm in the mood. This can not only calm us, but inspire us too.
Health
Although this can contradict 'Taste' sometimes, learning to balance both is important. Any good health expert, nutritionist, or doctor, will tell you that eating well, and exercise, will effect your mood. I hate being told this myself, as I hate gyms, but have found a healthy diet, and exercise routine that works for me. This helped me much more than I initially gave it credit.
Purpose
As others have mentioned already, a sense of purpose can help avoid the meltdowns. While it may not be possible to fix this overnight, working towards bettering ourselves will give us a sense of purpose. It can be as large as figuring out what to do with ourselves in our careers, relationships, etc., or as simple as learning something new; like cooking, or playing an instrument, or a new language, learning to dance, whatever tickles you fancy. Setting short term and long term goals can help, and don't just make one, make a few. Writing lists can help with goals, and the act of writing a list itself is therapeutic.
Vent
Although avoiding meltdowns is the desired effect, venting is important too, as it releases stress. Sometimes a small cry is better than a full blown meltdown. Talking about our issues, even if they may seem unimportant, can release much tension. Talk vent, cry, let it all out. This can also be done through exercise, or even simply leaving the room. You'll be surprised how therapeutic it can be to walk outside, or to a park, or go for a drive. The drive is one of my favourites, as it seems to work the best. If you can't drive, or don't feel up to it, take a bus/ ferry/ bike ride, whatever. Being creative is a form of venting too. Draw, paint, write; doesn't matter if you don't have the talent, just do it to feel good. Even if it's as simple as finger painting, scrap booking, taking photos, or paper mâché.
Hope some of that is useful. Look after yourselves
