My girlfriend got slammed with a high blood sugar and ketoacidosis episode about a year ago. We both did a lot of research, and she started using the following supplements as soon as she was released from the hospital (including 2 1/2 days in the ICU).
After one month, the doctor took her off insulin, she continues to take metformin, and she hasn't had an abnormal blood glucose reading since the third week after she was released from the hospital.
Supplements that helped immediately:
Fenugreek standardized for 4-hydroxyisoleucine for increased insulin secretion when eating carbs; Chromium, biotin, alpha lipoic acid, and banaba leaf standardized for corosolic acid for increased insulin sensitivity; and
Gymnema sylvestre and benfotiamine to reduce HbA1c.
Also, she used a combo of red yeast rice extract and niacin, with Co-Q10 to bring her triglycerides and cholesterol down, along with NAC and milk thistle to counter abnormal liver and kidney readings. Remarkably, at both the 3 month and 6 month mark, her blood and urine work showed everything to be normal. she's 51, but she's been a "T-Woman" for all of her adult life, so we believe that also had to have been a major factor in her quick recovery.
Other supplements that we believe have a positive impact on blood sugar metabolism include magnesium, carnosine, carnitine, cinnamon, bitter melon, mulberry, and apple cider vinegar.
As with anything on the internet, do your own due diligence - don't just take my recommendations as gospel. That said, I truly believe that these supplements can help a reasonably fit individual.