1. Check the battery.  Battery voltage should be:
• - 12.4-12.8V for a conventionnal battery (the clear type with liquid); 
• 12.6-12.8V for the newer MF batteries; 
• 12.8-13.2V for even newer Gel or AGM batteries.  A deffective battery cannot be recharged.
• Load test it if necessary.  To load test it you can use your motorcycle's starter by disabling the ignition and cranking it for 15 seconds.  In the end, that battery should still be at least 10.5 volts.
2. Check the charging or rectified voltage: 13-15V for 12 volt systems.  Rev the engine to mid range a few times & record the highest value.
• If the charging voltage is too high, this will dammage any battery.
• The possible causes are:
1. The regulator
2. The voltage sensing lead.
• If the charging voltage is too low, proceed to step 3.
3. Check the stator output.  Most street motorcycles use a 3 phase charging system.  Most with a magnetised rotor (PMS), some with an electromagnetic rotor (EMS).
• For the first kind, unplug the stator coupler & connect your voltmeter in VAC @ each set of stator windings running the bike @ idle.  You should get about 15VAC (3 times).
• For the ElectroMagnet System type, keep the coupler connected & do the same thing.  You should get at least 9VAC.
4. If this test is inconclusive, check the resistance of your stator (.2-2.5 Ohms).  And (very important) verify no connection to groud when unplugged. 
• For the EMS kind you'll also need to check the resistance of the field coil (2-10 Ohms) and also make sure it doesn't have a connection to ground (when unplugged).  That field coil will need to have battery power to it (when turning the key on) for the electromagnet to function.  It will aso need a good regulation (usually on the negative side) before the ground.  The regulator can be checked, by simply by-passing it.  When doing so, the charging voltage will rise (carefull, it may rise too much) if deffective.
5. If the stator output is OK:
• Check the rectifier
• Check the pos connection of the rectifier to the battery positive.
• Check the neg connection of the rectifier to the battery negative.
6. If all of these are OK:
• See if the idle is not too low.
• Check that there are no components that stay powered with the key off.
• Verify that there are not too many optionnal electrical devices.
• Last but not least, is the bike being riden long enough after each engine start?
7. On a Permanent Magnet System, the regulator cannot be cheked so replacing it is the last solution.

If this doesn't help you or doesn't give you enough details, leave me a message, I'll be glad to guide you.