I know this is really obvious, but I've been riding my 1150 for years with its sketchy headlights and never thought to do this. I've just upgraded the OEM bulbs in the dipped and high beam to Xenon ones, for the princely sum of £12 for the pair. And they do exactly what they say on the tin - they are twice as bright. Not fixed the problem completely, but a definite improvement and a 5 minute no tools job.