Keep them together. It is important that gifted students be able to move at their own (quick) pace as much as possible, so grouping students by ability for some subjects or having special gifted-student learning time is important.
Be open-ended. Ask thought-provoking questions, not yes or no ones. Assign projects instead of piles of worksheets.
Encourage questions. Gifted students not only know the answers, they have more questions on the topic. Instead of being afraid that you don't know the answers, use the questions as springboards for learning how to do Internet or library research.
Remember their ages. Just because the child in front of you can talk about quantum physics intelligently doesn't mean she won't get cranky when she's hungry. Brains don't equal maturity.
Incorporate their passions. Most gifted children develop deep interests in certain subjects. Use those interests as frameworks for teaching math, reading, science, social studies and writing skills.
Make sure they try new things. Most gifted students are not gifted in every area. Helping them step out of their comfort zones---whether it's encouraging mathematicians to write stories or sedentary readers to try sports---will make the students more well-rounded and empathic.