I guess the first question is how big is your budget?
Expensive ways- Carbon wheels, Carbon fuel tank, Magnesium engine cases. Then replace all fixings with Titanium ones.
Less expensive ways, dump off everything that is unnecessary. This will depend somewhat on your view of practicality, think mirrors and lights to begin with. Fitting rear sets with no rear seat or footrests will allow you get rid of the indicators as well. You could go further by getting rid of the charging system and run total loss ignition but you'd have to experiment on range.
Less expensive still, only fill it with 5lts of fuel instead of 10lts
And finally, possibly the biggest weight saving of all can depend on whether you are built like motorcycle racer Dani Pedrosa or Shirley Crabtree- definitely NOT a motorcycle racer.
I think Utopia will be along soon to give his thoughts on the biggest gains...I mean losses!