I can't give the best advice/response with this since I'm sure I'm not doing gesture drawing exactly correct; however, for the 5 to 10 minute gestures I do the initial pose and then I focus on where the light hits/curves of the light (such as for a lot of them around the stomach area you'll have the shadow around the belly button but before you actually get to the belly button area there will be light if that makes any sense. Basically the ab muscle thing). If you do the longer studies I'd suggest adding some color in the mix too and practice with that.
On how to start a figure drawing- I've seen people do boxes, circles, and simple stick figures. I personally just use what I use for an initial sketch for an illustration with the main focus on the torso and limbs rather than hands, feet, and head. Looks messy but I can translate what is going on so I call it good.