I have been at a loss at what to do to strengthen my hams. I can't seem to do curls with more than maybe 40lbs on a machine without getting WICKED cramps on about the 2nd set. It's freaking embarrassing to get stuck on a machine because you are screaming (inside) and can't straighten your leg.
I work on warming up, stretching different angles, and spend my time after almost all workouts in the Jacuzzi trying to stretch a little more.
It all started during AYSO soccer30+ years ago. I'd play midfield and run my ass off. Come home after game, and invariably one or both hams would cramp up. A double is a special kind of hell!
Supplements of potassium, more bananas and milk than I should consume, additional stretching.