Quick Strategy: "Thanks, but I'm allergic -OR- I ate before I came, you know what though, it looks delicious and I'm sure it is, please do offer (insert name of random person at party) some, they were complimenting your culinary talents earlier!"
Okay... So its not entirely the truth, but who's gonna know in a new place if you aren't really allergic to gluten or at least intolerant/dairy/whatever offensive food is being pushed on you. Heck, its not even a lie, very few people aren't intolerant to gluten/dairy/grossfrankinfoodridiculousness, we're not biologically composed to consume it.
I have found, generally, that the less of a deal you make about something, the less people notice. If you let it go with a "No Thank You" they'll let it go. If you go on about it, they'll get uncomfortable and label you as the one with the odd restrictions. Most of my friends didn't know I had gone primal until I flat out told them because I didn't tell them, and most of my family still hasn't noticed that I didn't grab a roll at supper or have mashed potatoes with my steak and salad.
Ultimately, no one really cares very much what you eat as long as you have a simple reason (i.e. eating before you came, having a big lunch, not feeling well, gluten intolerant) for not wanting to partake.
In a situation where alcohol offerings might be the rub, volunteer before hand to be the D.D. I've taken up this practice and have scored some pretty sweet thank you gifts, even though I kinda miss catching a buzz with my friends, I'm learning how to have just as good a time (if not more laughing at how dumb they get) without it.
I'm young and I'm dumb, but I navigate social situations pretty gracefully these days, I'm no Emily Post, but I make it through. No matter what, give you're most sparkling smile while saying no, razzle dazzle baby, no one will notice you don't know the words!
"It is not what you are that holds you back, it is what you think you're not."