To get a maximum mileage, try to drive in a constant speed (35-55Km/hr).
Try to maintain the Air pressure of tyres at 25PSI for Front and 32PSI for Rear. (Check once in a 5 days and in summer, check once in a 2 days) cuz, less air in tyres will have a more ground clearance and results the increase in friction for tyres with ground.
Since you are having premium bikes(100-125 CC), gear shifting should maintained in such a way that your engine does not get strained...
Try to drive your bike with less engine noise...