I agree with Michael's advice - I get a lot of potlucks in academia (scientists seem to love to cook...) and I find that water is a great tool.
I've also found filling up my plate with my own food first helps - I'll eat that and usually be full, or close to it. Carrying around my food on a plate is also a great way to ask others to try it! I've had instances where people start to ask me what's in it, and I've had some good discussions about low-calorie cooking (with my very limited knowledge!).
I've also found that by openly talking about my food choices, I get less people offering me to take home sweets, but YMMV on that, as I know plenty of other people who take it as an opportunity to 'fatten you up' when they hear you're having success losing weight
Barring all, a polite "no, thank you" without offering extra explanation DOES work. I know I always tend to want to elaborate, but then you're just giving them more to work with when they want to give you food! Sometimes I'll also say "Next time I may try something of yours. I think my eyes were bigger than my stomach! I'm full!"
Female, 31 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/27/14 = 180.4 (net: -34.8)
Current mini-goal: 180 lbs.
Next large goal: 165 lbs. by 12/25/2014
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."