It's very important for you to monitor how your dog responds to the injected insulin dosage. You do that by testing the urine with test strips usually available at drug stores or pet shops. The test strip will tell you how much sugar is present in the dog's system, and you may have to adjust the dosage of insulin based on the results. Have your vet teach you how to use the test kits, what results to look for, and when to administer higher or lower dosages.
Keep a record of the test strips results, amount of insulin given, and your dog's eating behaviors and attitude. This will not only help you understand the dog's condition, but will help your vet if other problems arise.
http://www.pgaa.com/canine/health/diabetes.html