Is the battery getting charged? ie voltage with motor runing at speed is over 13 volts, and run for more than 5 minutes, and not discharged when stoped (leaving lights, ignition, heated grips (err Africa ... ) on).
Are the connections tight and clean between battery and starter motor?
For the battery as well as the capacity (Ah ampree hours) you need to consider the CCA (cold cranking amps) rating.
What temperature are you riding in? (Could this be heat related - ie battery, motor and starter motor all gettting very hot?) Would cooling (say a spray of cool water over the motor before you leave it parked) help?
Think you need to provide more information to get a more focused suggestions. Good Luck.
Regards Frank Warner
motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S