For starters, curb hamming and posturing by taking pictures inconspicuously: Use the light in the room instead of the ultra-obvious flash, and get people in mid-conversation instead of posing. If you do want posed shots, avoid trite announcements like "Say cheese!" -- you might as well say, "Fake smile!" Instead, tell folks to take a breath and relax their shoulders, since cameras tend to make people tighten up.
Once you've taken the picture, take another. You never know what you?ll catch, and that goes double for angles (which you should vary) and framing (which you should play with). Try holding the camera slightly higher than eye level -- so that guests are looking at you, not directly at the camera -- or shift to one side to add a sense of motion. What does that mean? Put it this way: Your laughing boyfriend is a good shot, but your laughing boyfriend beside a friend who's just upended her eggnog is a classic. It might even be your next Christmas card.