My dog has a few ways. He will try to position himself so that my hand ends up on his sheath. If that fails for a long time, he tries to mount my leg or arm. And if that fails he will vocally complain about it.
His 'proper' way of asking for a suck is after we play for a while (I guess he sees it as foreplay), he will come to me and place his paw on my leg.
Then we have sex and when he has had enough, he lifts his left hind paw up to signal that. This usually happens as he is loosing erection. Those gestures are so precise and happen in very specific situations that I have good confidence I know what they mean.