Triceps Brachii is a muscle located in the back of the arm and makes up about two thirds of an arm's volume. Many people who want big arms make a mistake of working their triceps exclusively. You should always include an overhead triceps movement in your training program to target the long head, its the only way to work that head. In doing triceps exercises one must always remember not to let any part of the arm above the elbow move.
Shoulder joint remains stationary at all times.
Dips can be done weighted or regular. To hit your triceps and not your chest with this exercise, elbow position is crucial. Elbows should not be close to your body but instead pointing outwardly. You should go as low as possible and the slower you go the more effective it becomes. Lower the bar all the way down until the back of your hand touches your forehead and push it back up.