while you can lose some mpgs due to mechanical problems, the biggest factor is how you ride.
take off a bit slower, keep the engine in the lower rpm range, coast more down hills, etc will get you a bunch more miles for each tank.
that being said, if everything IS stock, make sure it is maintained. i gained 3-5 mpg from adjusting the valves. make sure your carb is tuned correctly. see if your idle speed is too high (but you don't want it too LOW either, as you need to maintain oil pressure).
make sure there are no exhaust leaks or gas leaks...
you could also do a compression test on the cylinder head to make sure it is in sound shape.
aerodynamics come into play a lot on motorcycles. wearing leathers (or at least not clothes/jackets that flap in the wind) will help. taking off saddle bags will help too.
i'm going to go ahead and say that if you a TRYING to get good mpg and not riding at 70-80mph, then you should look at the more maintenance/mechanical culprits (carb, exhaust, valves, etc).
oh, and as a pretty in depth way of getting more gas mileage, like James May said, converting from a belt to a chain drive with the right ratio will make it more fuel efficient (but you'll lose off-the-line acceleration).
now, keep in mind this is coming from someone who rides like a bat out of hell and gets closer to 30 mpg because of it, with a cone air filter, rejetted carb, and jardine exhaust. for me, the thrills of speed are worth more than a dollar or two in gas, but not everyone shares the same paradigm