Insulin is not only useful for competitors. Many athletes use it to increase training frequency: insulin increases glycogen uptake by the muscles which accelerate recovery. Plus, since it's anti-catabolic, it can reduce the negative impact of each training session an allow you to train more often and harder without overtraining.
Insulin can be used as follow:
1) For recovery optimization: for many type of athletes, to increase the rate of recovery after a training session. This strategy requires the athlete to take 5-10iu after each training session, along with 75-100g of protein and 100-150g of carbs.
2) To increase glycogen storage for sporting events which depends on the availability of intramuscular glycogen (e.g. 200-800m sprints). The athletes can use insulin before a training session or before a big event to bulk up their glycogen stores. This can be more tricky, but 5-10iu 3 hours before an event along with two 'meals" of 100-150g of carbs (1 meal immediately after injection, 1 meal 60-90 minutes later) is a good place to start.
3) To increase muscle mass. This is most useful for bodybuilder, or athletes lacking muscle mass. By using regular insulin injections (2-3 per day, must be fast-acting insulin, preferably humalog) along with a high carbohydrate and high protein diet. The frequent injections reduce the catabolic response to training, which leads to greater muscle gains. Insulin also helps with glycogen and protein muscular intake which facilitate protein synthesis.
IMHO the best way to use insulin is to improve recovery post-workout. This is the safest method of use and the one which can potentially give you the most results.