Plan ahead.
Work out coping strategies you can put into action BEFORE the stress goes through the roof - e.g. go for a breath of fresh air outside or take refuge in the loo if the reception is in a posh hotel (one wedding I went to, the ladies washroom even had armchairs and some magazines...strongly tempted to hole up there for the duration!)
Think of the worst that could possibly happen and then ask...well, how bad is that?
Remind yourself that everyone else is probably far too busy stressing about how they look/feel/sound to spare any thought for you!
I'll never forget envying another girl for being so confident and poised in social situations...until the moment she turned to me and asked me how I managed to be so confident in social situations!!!
And all the time, I was convinced I looked like a complete idiot, nervous, shaking, tongue-tied and red faced! I've never forgotten that lesson, and it really helps to remind me that everyone else is probably feeling as bad as I am!