I suggest that you work on tuning first, really. See if you can lean it out a bit, if you have a proper way to monitor your engine. Driving technique is usually the kicker. Get 'er into high gear as soon as you can, and if you have to accelerate, don't downshift. Load the engine up as much as you can without it stuttering.
Swap your oil for something thinner, if you're comfortable with it. Of course, driving slower will also up the mileage for you. Remember that any time you're sitting still, or the clutch is disengaged, your engine is burning your money and doing nothing in return. Turn it off if it's not driving the bike. Idling is bad for air-cooled engines anyway, in most cases.
If you're really interested in doing aerodynamics work, you can start by adding functionality to the bike. If you never put anyone on the back of it, you can trim down the seat, add a trunk (make it yourself, if you can) and taper the shape back so it forms a cowl/backrest that slopes into a sort of fish tail. You can add side bags, tank bags, etc. so that you can carry stuff in the space that an aerodynamic shape will occupy, and those things, if smoothed and tapered properly, will help the flow of your bike as well.
Beyond that, you can begin adding a framework for some body panels just by bending up old wire coat hangers into a frame, then stretching some window screen over them. It'll look like hell, but it'll get the job done. At high speed, air passes over a tight screen, rather than through it. Leave a cooling area for your engine, and don't cover up the exhaust too much - gets hot around there.
I suggest you check out Craig Vetter's site for more ideas and inspiration about aerodynamic forms and motorcycles.
Craig Vetter Fuel Economy better mileage
