The only way to gain strength in your arms is to work them out. And this "dumbbell tricep extension" from Men's Health is just what you need. This exercise is aimed at isolating the triceps muscle of the upper arm.
Sit on a bench holding a heavy dumb-bell in your left hand. Extend your arm straight overhead with your palms facing forward and place your right hand on your left elbow for support.
Without moving your upper arm, lower the weight down in front of your face until the end of the dumb-bell touches the top of your chest in front of your shoulder. Finish the set before repeating with your other arm.
This exercise is aimed at isolating the triceps muscle of the upper arm and is best kept for later in the workout when you have completed the larger compound moves such as the bench press and bodyweight dip. Please take care at all times to avoid hitting yourself in the face.