Here are some things I've learned from experience about taking better photos of food.
1. Don't use a flash. In a restaurant setting, it's distracting to other diners. It will also generally make your food look garish.
2. Don't shoot from a bird's eye view. Get down and dirty with your food.
3. In low lighting, a miniature tripod, beanbag, or perhaps something on the table can be used to steady your camera and get a crisp shot.
4. Find something white to include in the shot, be it a plate, table cloth, or piece of paper that you hold up. Make sure your camera includes the white in its metering, and you'll often end up with a brighter, clearer shot.
5. Try to sit at a well-lit table, or visit restaurants at lunchtime or in the early evening when you can take advantage of natural light (depending on the restaurant's setup, of course).
6. Compose your shot carefully. Usually, you're trying to fit a round plate into a rectangular frame--you start out with a challenge every time. Pay attention to the things that sneak in around the edges of the plate.
7. Zoom in (either with your entire camera or just the lens) to keep the focus on the food and away from the silverware, candles, and your friends in the background.
8. Sometimes a friend's solid-colored shirt can make a great backdrop.
9. Take 3-5 (or more!) photos of each subject.
10. If your camera has a setting that you can adjust depending on what kind of light you're in, use it! It will make a huge difference.