Could be that your clutch is over adjusted so its too tight or even not tight enough... i would investgate this before... looking into gearbox problems... hitting neutral is a common problem for some riders due to riding style.. or having a machine you are not used to... it often reqiures you to fine tune the clutch to make it easier for you to ride and to suit the way you ride the machine...
If its a sudden problem thats occured on the machine then i would look into a mechanical issue but try the clutch adjustments and simple things like that first!
Chris, March 2009