Don't go to a party hungry, if you are dieting. You will not be tempted by the unhealthy snacks and desserts that are normally available at a party. You should consider diet drink options as well, using diet coke in mixed drinks is a great way to cut calories.
Some people find that when trying to lose weight, their tape measure is a better ally than the scale. If you're a woman, measure your waist and hips around the widest part once every week or two as you make progress toward your weight-loss goals. Seeing the inches go down will inspire you to reach your goals.