Generally, wheelchair users can focus on resistance exercises to improve your upper body strength and help reduce your chances of injury. You should always talk to your doctor or physical therapist to get clearance and guidance for your best options. One place to start is with videos (check out Collage Video for ideas) you can do at home or workouts, such as this Seated Strength Workout or this Seated Upper Body Workout.
If you're interested in doing more, you might want to think about investing in some specialized exercise equipment. There are many new strength training machines available for people in wheelchairs, as well as hand-cyclers and other cardio equipment. But, don't let a lack of special equipment keep you from your work out. If you have upper body mobility, try lifting your arms straight out in front of you, hold for a few seconds then lower. Next, lift your arms out to the sides (stopping at shoulder level), hold, then lower. Do both of these exercise 15 to 20 times and, as you get stronger, hold light hand weights. More specific upper body activities include shoulder shrugs, overhead presses and bicep curls.
If you're competitive, another option is to try organized sports. Wheelchair Sports can help you find events near you that include basketball, archery, fencing and more. If you need help with training for a specific sport or event, the NCPAD can help you find fitness programs in your area.
Stretching and flexibility is is important too for reducing the chance of injury. Specifically, you should be stretching all the major muscles in your upper body, including your shoulders, arms, back and neck. Exercises for Wheelchair Users offers examples of exercises and stretches you can do for your upper body and includes tips on proper form.
If you have a disability, you have to work much harder and be much more creative about exercise. Talking to your doctor, physical therapist or other experts can help you find activities to keep your body strong and active.