When I first got my bike I burned up 2 bulbs, and said enough of that. So I bought a Moose voltage regulator, because the 89's didn't come with them. Haven't had to replace a bulb since then. If you don't have one, get one.
I can check my service manual this weekend for voltage/resistance requirements, but what speedy said should work for 89's too. Sounds like the lighting coil is bad. A new high-output coil (70 watts) from dennis kirk is about $80. Might be a good investment, even if thats not your problem.