As said above, check your battery cables for good connections/damage. They must be CLEAN and TIGHT.
Another common point of failure is the plastic connector between the alternator and the bike's wiring harness, located about 6 to 8 inches from the alternator. Moisture gets into it and causes corrosion/high resistance. Easiest fix is to cut out the connector and just solder the wires together. There is no need for a quick-disconnector in a circuit that is likely to be disconnected and reconnected perhap twice in the life of the motorcycle.
Also, suggest remove and INSPECT every fuse and associated retaining clips in the fuse box. I had a fuse socket clip get loose once, and the resistance caused it to overheat and melt the fuse box. Since then I carry a spare fuse box assembly on the bike at all times. It takes about two minutes to swap out the whole thing if it quits away from home. These fuse boxes have a reputation also for bad solder joints cracking and causing electrical problems. Search the forum for threads about resoldering the fuse boxes.
Even if there is some other cause to your problem, like just a bad battery or failed alternator, fixing the above system weaknesses is a good idea to prevent future problems.