I agree with most of the above - saying that you're dieting for whatever reason tends to make aggressive or defensive responses. I also just say "Oh, I'm full, but thanks!" about desserts with friends, or saying something like "Trying to cut back on chocolate/white flour/sugar/fats/etc." (whatever excuse seems valid) also does help. They may say "Oh, then just get the ice cream!" which is why "I'm full" often works for me.
If they're good friends, I find that a very simple and polite "No, thank you" often works, and I'm trying to do that more often (because I don't need to make excuses for myself!)
If someone asks me why I'm dieting or changing my lifestyle, I just say "for my health, and because I feel better when I eat this way." There's no real witty comeback that I've found for that one from people! You don't need to give any other reason than that! Good luck, and I hope your friends are supportive!
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/30/13 = 179.8 (net: -35.4)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."