This exercise is great for toning the back portion of the arms (it can also be used in combination with other tricep exercises to double it's effectiveness- which we will go through later)

Movement:
Stand with your feet hip-width apart and your knees slightly bent. Grasp a 10lb dumbbell (feel free to go heavier or lighter depending on your individual ability) by the inner plate or body, with both hands and raise it directly overhead. Keeping your elbows squeezed tight to your ears, lower the dumbbell behind your head by bending both elbows. Return to the start position and repeat. Aim for 12-15 reps.

Note: Be sure not to over arch your lower back while performing this exercise. If you are unable to prevent your lower back from arching, use a chair with a back rest for support (make sure, however, that the back rest is low enough so that it will not get in the way of your dumbbell)

Focusing on the triceps more today. We will be looking at an exercise called the 'Tricep Dip'. This exercise is fairly easy to do and requires no extra equipment other than a chair which can be readily available at home or at work (if you have a closed office space ofcourse - or not, if you're that free at work).

Movement:
Sit on the edge of a bench or chair (preferably one without arm rests or arm rests that do not restrict movement) with your feet flat on the floor, knees bent 90 degrees. Grasp the seat on either side of your butt. Walk your feet out slightly and inch yourself off the seat. Bend your arms, keeping your elbows pointed straight back as you dip your butt towards the ground. Lift your body and butt back upward, and hold this position (this is one rep). Concentrate on using your arms to raise your body, rather than pushing up with your legs. As you continue you should feel the contracting and relaxing in your tricep muscles. Do 10 to 15 reps.

Note: You can circuit this exercise with the standing tricep extension for added 'work' for your triceps, and to get better results. 
Also be aware of the distance of your feet from the chair; the further away your feet are from the chair the harder the exercise will be, so start off closer and gradually work your way to the harder positions.