Usually by penetration as the culmination of a gradual build-up of speed and depth of thrust, preferably she is responding, looking into my eyes and willing me on verbally...this is generally in mish or cowgirl, cowgirl being great to get a finger on her starfish to push me over the edge.
A few memorable occasions a girl has sucked it out of me with just her mouth and minimal hand action, maybe just gripping the base, that felt great too, esp with the eye contact.
Just recently I've managed it a few times in doggy, but the lack of eye contact and boobies jiggling makes it more difficult.
Of course, if things are slow, it's a HJ as last resort.