How can I say "no" to desserts? (see details)?

When I'm motivated about being healthier/losing weight, I tell myself that I will stop having desserts (I don't have them too often, anyway, but lately it seems like they keep showing up at parties and such that I attend....). But then, when a dessert is in front of me, or someone offers some to me, I can't help but have some. It's so tempting because my friends are often eating cookies at lunch (sold in the caf itself), my cl will have cake around, or my sister will decide to whip up some delicious-smelling baked goods on some random night. Since I'm so tempted, I just go with it, but then I feel really guilty later. I can never seem to stick to my "no dessert" diet though. Any help? :)