This alone will dwarf every other single weight saving you can make.
Originally Posted by DoWorkSon
Fill tank with less fuel?
When you're riding with people on "proper dirtbikes" remember that they only have half, or even a third of the range of your bike. I find out what bikes other people are planning on riding and aim to be at the start (even if that is a gas station) with about 10 miles more range than the shortest-ranged bike.
The "F800GS Wunderlich Light" (Google it) is a good example of a really shaved down F800GS. It's not very pretty and it'd cost a lot to build one, but worth looking at for inspiration.