To reduce the risk of hypoglycemia if you have diabetes, follow a regular routine of exercising, eating your meals, and taking your medicines at the same time each day.
Prolonged or strenuous exercise can cause your body to produce adrenaline and other hormones that can counteract the effects of insulin and cause your blood sugar to rise. If you are participating in strenuous exercise (exercising at your maximum capacity) or prolonged exercise (lasting for several hours or more), your insulin and/or oral diabetic medicine may need to be changed. Talk to your health care provider about how to adjust your medicine.
Be careful exercising when your medicine is reaching its peak effect.
Depending on the time of exercise, reducing your dose of either long-acting insulin or short-acting insulin will be necessary. Your doctor can recommend how to make this adjustment.
Exercise with someone who knows you have diabetes and knows what to do if you have a low blood-sugar reaction.
Wear a medical identification tag (for example, MedicAlert) or carry an identification card that states you have diabetes.

The American Diabetes Association offers these basic exercise guidelines for those with diabetes:

Discuss with your doctor what types of exercise might be appropriate for you. Complications of diabetes such as severe eye disease and nerve damage may make some forms of exercise dangerous for you. Your doctor may also schedule a test to see how your heart responds to exercise.
Do not exercise if your blood sugar is greater than 250 mg/dL (milligrams per deciliter) and your ketones positive. This is an indication that you already may have a lack of insulin and exercise will only cause a greater rise in your blood sugar. Hydrate yourself and adjust your insulin as necessary, contact your health care provider.
Use caution when exercising if your blood sugar is greater than 300 mg/dL without evidence of ketones, exercise may help decrease your sugars, but it's possible they will increase instead. Hydrate well prior to and after exercise and keep track of your sugars and ketones.
Learn the effects of various types of exercise on your blood sugar.
Have carbohydrate-based foods available for exercise and for the period following exercise. Add carbohydrates to your meals if you plan on doing exercise, adjust your insulin dose appropriately in anticipation of exercise.

If you have diabetes, check with your health care provider before you begin a an exercise program. Tell your doctor what kind of exercise you want to do so adjustments can be made to your medicine schedule or meal plan, if necessary.
Start slowly and gradually increase your endurance.
Choose an activity that you enjoy. You'll be more likely to stick with a program if you enjoy the activity. Make exercise a lifetime commitment.
Consider a water exercise program. Some other exercise options include walking, riding a stationary bicycle, swimming, or muscle stretching.
Exercise at least three to four times per week for 20 to 40 minutes each session. Ideally, you should exercise every day. A good exercise program should include a 5- to 10-minute warm-up and at least 15 to 30 minutes of continuous aerobic exercise (such as walking or biking) or muscle stretching exercises, followed by a 5-minute cool down.
Wear good shoes and practice proper foot care.
Drink water before, during, and after exercise to prevent dehydration.
Do not ignore pain -- discontinue any exercise that causes unexpected pain. If you continue to perform the activity while you are in pain, you may cause unnecessary stress or damage to your joints.