I feel for you. First, take all the advice you can get and then leave it all. You'll have to work out what helps you since it seems to vary from person to person. Don't let it get you down if a few things you try don't seem to work.
If you can get away, splashing water seems to help. If you can't get away, talking to yourself as inconspicuously as possible and telling yourself that "you're okay", "you're doing well", anything positive as I saw already posted. Sip water if you can. Sing to yourself. Again, all this is only helpful if you find it will work for you...and if you can do it without feeling like everyone's watching you while you do it.
Mundane things are good...at home I find that brushing my hair, teeth, something totally routine helps bring me back a little. So, if you can fiddle with an object you have with you, examine it, maybe a worry stone? I wear a lavendar jade pendant quite often when I go out and I'll touch it and run my fingers over the carving.
Unfortunately in my own case, if I'm going, I'm not getting back easily. When I drive I sing to a CD. When out in public I prepare myself by thinking about it before hand and finding where the restrooms are immediately. If nothing else, it gives me a place to go to escape for a few minutes. I'll sit at the outside of a room rather than in the middle. so, get your back against a wall if you can. If you can find a locket to put some lavendar oil or something else soothing in it to smell, that's good. You want to activate anything that is other than the anxiety and stress.
Lots of things from everyone to try out. If you discover something that works for you that no one else mentioned, I hope you'll share.
updated by @gigi-miner: 01/21/17 07:44:15AM