Like Bryan said...
You can try to balance things out a little better, but that's about all. Use HID headlight instead of halogen for more brightness and even less current than the stock headlight, that's a start. Make sure the voltage regulator's contacts are clean and shiny, like new points: that's another. Make sure the GREEN grounds are clean, everywhere, especially at the frame. Install a #1034 tail bulb, as it was the original type (today's #1157 non-Stanley bulbs use considerably more current). Disconnect the front running lamps in the turn signals, that can help.
All the little things together make a significant difference! When the bkes were new, they did OK with the systems: today, you will find that the batteries from most cut-rate joints are far poorer quality than, say, Yuasa or a good AGM type of battery, and this causes poor storage and erratic field current in the alternator, making less output overall. A good battery for these bikes will cost over $50.