During tournaments I've had these warming pads, they work decently but not well enough and I can't warm my hands in the middle of games.

I live in a warm country but the AC makes my hands really cold. I usually just warm my hands up by rubbing them on my legs. Works decently..

Hands in between my legs or I sit on them to warm them up. Sitting on my hands warms them up good. But I can't game like that though.

Large space heater to my left gets the ambient room temp to ~70 (F). Small space heater to my right points directly at my mouse & keyboard to keep my hands warm, specifically. Works OK, but not optimal.

Whenever I get cold hands, I'd take a break and do something which requires me to move around more than I'd do sitting still.

Cold hands are a huge problem. I stick my hands between my thighs, usually while dead lol.

I rinse my cold hands in hot water for some time, helps for the moment but cold hands usually come back really quick, especially in winter times.

A stress ball to warm up my hands and fingers when I'm sitting at the computer.

I put gloves when I have cold hands. I've modified these gloves slightly to be useful for gaming. They aren't comfortable for gaming but their comfortable for slightly warmer hands.

I warm my hands between my thighs or armpits. Otherwise I take really hot showers before important games. 

I use warm water or a cup of a tea to hold between games but only solves it temporarily. 

I have an electrical hand warmer by my desk which I use commonly. I don't even have that cold hands but just slight cold hands makes me uncomfortable when gaming.

Pushups. Some go wash their hands in cold water but that is only temporary. Pushups gets the whole body going and improves heat distribution in the whole body, hands included.

I take a short break and get a warm drink. Holding the warm drink helps.

Warming up my hands by just rubbing them, turning on a heater, or playing until they're warmer.