While the ergonomic triangle of a sportbike-and sometimes sheer laziness-makes it difficult for us to use our lower bodies to help with various riding aspects, there are many ways in which using your legs and torso can help you ride quicker, smoother and safer. Performance riding requires that you use all the tools in your skills toolbox, and lower-body use is key in several ways.
Grip pads on your tank will...
Grip pads on your tank will help you use your lower body effectively; you'll be applying lots of force with your inner thighs, and don't want things sliding around.
The biggest and most difficult task for your lower body is supporting your upper body, leaving it free to steer the motorcycle. The clip-ons are for steering only, and any additional input-either from bumps or just holding yourself up-will upset the chassis. Ideally you want to support yourself by gripping the tank with your legs and using your trunk muscles, to the point that you have no weight on your arms-even when you are tucked in on a straight or hanging off in mid corner. The benefit here is twofold: The weight of your arms and upper body is not affecting steering, leaving you free to make minor corrections. And, in the opposite sense, if your arms are relaxed and loose they will not have a tendency to add unwanted inputs into the front end that can upset the often delicate balance of steering geometry and result in bump deflections being amplified to the point that a "tankslapper" (where the steering whips back and forth violently) results.
It may seem impossibly difficult...
It may seem impossibly difficult at first, but ideally when you ride your entire upper body is supported by your lower body, not by your hands on the clip-ons.
With your bike on its centerstand or a paddock stand (have a friend close by if you attempt this on a sidestand) sit on your bike in a comfortable riding position. Grip the tank with your knees, clench your abdomen muscles, and try to take your hands off the bars. At first you may find that you simply cannot without falling on the tank. This means you need to build those trunk and calf muscles with sit ups and squats until you can hold on with just your lower body. Lean into a bit of a crouch and try again. Work on this while riding (um...don't actually take your hands off the bars though), at first on straights, then in turns. Eventually you want to be able to have no weight on your hands-even when hanging off while cornering at the track.
Another job for your lower body is to act as suspension. Watch a BMX or motocross race, and you'll see those riders standing on the pegs, bending their legs over whoops and bumps to help soak up the hits. While you don't want to lift your butt right off the seat (except in extreme cases), you can help absorb bumps by sharing your body's weight between your feet on the pegs and your butt on the seat; again, support yourself with your lower body and let your upper body move in relation to the bike (or not move relative to the direction of travel, if you consider that point of view) and this will reduce the strain on you and your suspension.
Pay attention to how racers...
Pay attention to how racers transition from side-to-side and you'll see subtle differences in how they lever the bikes with their lower bodies, using whatever is available to put force on the chassis.
Your lower body can help when it comes to turning your motorcycle as well. When transitioning from side-to-side, push on the inside footpeg; you can also use your outer leg's inner thigh on the tank and heel on the footrest hanger to help "pull" the bike over. You want to use these forces to rotate the bike around its roll axis, using the rest of your body as leverage to aid the act of countersteering. Check the manner in which various racers transition their bikes from one side to the other and you will see the subtleties required when hanging off at the track: when the bike is rolled, when the rider moves relative to the bike, and when both move together. Experiment with different techniques to find which works best to get your bike turning the quickest.
While you're not moving around...
While you're not moving around on your bike on the street as much as on the track, you can still use your lower body to help steer, turn and absorb bumps.
It can all get rather complicated and involved trying to juggle all these aspects; unweighting the upper body, absorbing bumps and turning the motorcycle. Throw in hanging off in corners and tucking in on the straights and it can be a lot to think about. Start with the most important task-keeping a relaxed upper body by supporting yourself with your legs and torso-and add in the other elements from there. As with anything related to riding position, there is a balance between comfort and performance; try variations until you find compromises. It's not much good using a technique that gives you sore legs after just a few minutes. Most importantly, experiment! There are a lot of things to push on, not just the footpegs, and you may be surprised at what you can use to help you.