Perhaps your doctor needs to increase the dosage. I know that it is important to have a balanced consistent diet. Since you've mentioned that you have your diet under control you are aware of this. I find that eating the same approximate amount of carb during each meal everyday also greatly helps. Remember Metformin isn't a miracle drug, there needs to be work done on your part as well such as exercise at least 30-45 min a day.
I have recently begun a blog
http://www.mytype2diabetesmanagement.com/ where I will be posting about my diagnosis of being type 2 diabetic, my experiences and how I have managed my glucose levels. I will post things I've done that were successful and I'll also post my failures. Visit my site, it's in the infancy stage but we can learn from each other.