Grooming is a great time to get some exercise and develop the bi-lateral coordination and symmetrical strength that you need to become a better rider. Many riders have a strong dominant side--which can mean that the horses they ride also have a strong and weak side or direction. Strengthen right and left by grooming with both hands at once--you'll strengthen both arms and make sure you can cue your horse with symmetrical strength.
Start with duplicate brushes: two curry combs, two stiff brushes and two soft brushes. Put a currycomb in each hand and start your normal grooming technique, using both hands in a "wax on, wax off" motion. If you normally make circles with the currycomb, make the same circles with both hands, starting with circling both hands inward then both hands outward. You'll repeat the two-handed process with all your grooming tools.
Pick up your stiff brushes and repeat the process, using both hands equally as you flick the dirt out of your horse's coat from his ears to his tail. Finally, you'll use the soft brushes to bring the shine out in your horse, nose to tail. If you are right handed, make sure you use your left arm at least as much, if not more than the right; and visa-versa.
Double grooming will help you build strength on both sides of your body and will develop your coordination as well, making you more ambidextrous in the saddle. The added benefit is that your horse will get twice as much grooming and his coat will gleam.
For more strength-building exercises, check out www.JulieGoodnight.com for volume three in the five DVD riding series, Perfect Practice: Exercises to Improve Your Riding.