Berberine is arguably the best over the counter supplement to improve insulin sensitivity. There are other things to take along side of it as well...
Cinnamon extract has some anti-diabetic effects, proving to aid insulin function by a significant degree, potentially can help to improve your blood lipid profile and can reduce blood glucose levels...
Fenugreek has some clinical research that shows itt can reduce blood glucose
Alpha Lipoic Acid can help to decrease blood glucose
Also chromium can regulate blood glucose, insulin metabolism, and can encourage insulin sensitivity in the body
Berberine is the best of these but all of them can certainly be helpful