Choose the weight so that you can perform 8-10 repetitions in one set. The repetitions should be heavy, but not so heavy that you have to ruin the technique. If you start to move your shoulders or swing your whole body and loose the stable form, take a lighter dumbbell and try again until your positioning and technique is correct. Perfecting this technique is key for muscle progression, as if your technique is not correct, you are working more muscles than intended and this could lead to an injury.
Do Dumbbell French Press once a week, alternating with other triceps exercises: stretching your arms on a block or with dumbbells in a tilt, dips on the parallel bars. Also, periodically change the version of your exercises to load all of the muscle fibers as well.