The most important head to target for triceps thickness is the long head. EZ-bar French presses nail the long head, but they're easy to do wrong.
The force angle needs to be exploited with minimal joint stress. Doing them in this fashion will keep constant tension on the long head without shifting stress to the elbow and shoulder joints. Don't come to a complete extension of the elbow; that's not the long head's job. Higher reps in the 12-15 range work well. Expect a big pump.
- Grip the bar with a false or thumbless narrow grip.
- Slide forward in an upright seat or set the adjustable bench one notch shy of 90 degrees.
- The elbows should not flare out. Make sure they face in towards each other as much as possible.
- Let the weight of the bar pull the elbows backwards (towards the wall behind you) and then "drag" it forward while pressing up over the back of your head.
- Use a controlled negative and repeat.