Ideally, the height of the surface you are pushing against should allow for complete range of motion, without over stretching. Keep the shoulders down rather than allowing them to hunch, and do not bend the elbows lower than 90 degrees. In addition, avoid fully locking out the elbows.
Tricep Dips can be performed by bending the knees instead of keeping legs straight. Doing so brings the feet closer to your body, making the exercise easier to do. If bending the knees, try not to use the leg muscles to push yourself back up. If you are trying to make the exercise more challenging, the feet can be elevated using another chair or bench. Doing so will increase resistance significantly.
Keep in mind that not all exercises are right for everyone. Those with rotator cuff, elbow, wrist or shoulder problems should avoid doing dips and check with their doctor before attempting this exercise. If you are already including exercises to work the triceps, take this into consideration in order to avoid potential overtraining.