What is the best way to improve firmness of an erection?

Improve your health. Exercise, good control of diabetes, avoidance of certain medications, stop all tobacco and alcohol products, all of them. Antidepressants and betablockes are often cited as causes of reduced erectile function. Medications like viagra (sildenafil) can help, if appropriately prescribed by a physician. Exercise and no alcohol probably in my younger patients are my best recommendations.