Give this a try:
• Tricep kickbacks – Kneel on the floor or a bench with one knee in front of the other. Use one arm to support on the front leg or the bench and hold a small handweight (or bottle filled with sand or water, tin of beans) in the other. Position the arm holding the weight so that the upper arm is level with the back and the elbow is tucked in to your waist. Maintain this position and extend the elbow until the arm is straight, but avoid locking the elbow joint. Return to the start position. Try around 15 of these. By the end aim to reach fatigue so you cannot do any more. Repeat on both arms.
• Tricep extension – Stand tall with the spine nice and long, the tummy tight and the knees soft. With a weight in one hand, take the arm up to the side of the head and hold it in position with the other arm. Allow your arm to bend, under control, so that you lower the weight down behind the head and in between the shoulder blades. Extend back up to the start position. The upper part of the arm next to your head should be vertical throughout. Again try around 15 repetitions to fatigue on both arms.
Personally, I go swimming 5-6 days a week, and it really helps!!