Figure 8's are good.
Try varying the size and shape of the 8, use old car tyres, rocks, lumps of dirt, whatever, try not to create something that will mess you up if you clip it though.
Make the corners sharp, then do the 8 as fast as you can, then make the corners open ( sweeping ) and do that as fast as you can.
Look at what you want to improve on ( braking, powering out, sliding etc. )
and then work on that.
What I think lapping the 8 allows you to do is push it until you find your limit,
without the problems of being on a moto track with riders, jumps etc.
Also, try and be smooth, keep it tight, dont stomp on the brakes, no big hand fulls of throttle, smooth is surprisingly quick.
I think having only two corners and them being close together allows you to become familiar, with the bike and the surface, very quickly.