Do you
fully charge the battery when it dies or do you just boost it?
Motorcycle batteries require a deep cycle charge to get back to their full potential, or atleast that's what I've noticed.
When it does die, or next time before it does, hook up the charger (not trickle charge) for atleast 2-3 hours. This will give your battery a nice clean charge.
Also, is you battery stock from 2002? If so, it might simply be time for a new batt. They usually don't last more than 4-5 years....and it's already been atleast 4.
Try out the charger and let us know.