The key is to always be prepared. I have food stashed in my car and my purse for all occasions and that way I don't catch myself starving and I can offer some yummy veg food to my friends and they can try it and know that it's not too bad! My family is all non veg but when they see the stuff I make for myself, they always think it looks great and have insisted that I cook dinner one night a week for them! Also, buy the book "the joy of vegan baking" to make stuff to bring to parties. I used two of the muffin recipes for a friend's bridal shower and they went like crazy. Literally everything I've made out of there my dad has said, "this is the best zuccini bread, chocolate chip cookie, bran muffin I've ever had!
Another thing that works for me is saying "I don't eat meat" or "I don't eat animal products" some people get really defensive if they hear the word vegetarian or vegan and their guard instantly goes up. A lot of people just don't understand and it helps to be really nice and to not talk down to them. My dad is a hunter, most of the men in my family are and although it's not for me, I don't have as much of a problem with it as I do with people that go to the grocery store and buy meat that has been raised in a factory farm. It's better for the environment, not full of chemicals and antibiotics and diseses. I think if you're gonna eat an animal, you should have to be the one to kill it... people are just too detached from their food sources!
To sum it up, just be incredibly nice, smile at your waiters and hosts and be sure thank them. Always be prepared with yummy foods and never expect people to go out of their way for you. You're not gonna make them a meat dish when they come over to your house are you?
Ohhh, one more thing, because my response wasn't long enough... Don't say "I CAN'T eat that", it just perpetuates the sterotype that a vegan diet means you can't eat anything. You can eat whatever the heck you want, you're choosing not to though. People seem to be less defensive if you use phrases like "I don't eat dairy" or "I choose not to eat animal products" It might seem small, but to me it really seems to help people to be more accepting.