The trick is often to just get going. The bike warms up much better under a light load rather than sitting idling. Once it's running smoothly, even if it won't rev much, getting moving down the road (being careful with the clutch to not stall it), can be all it needs. Check out this thread on how different ninjette owners warm up their bikes:
http://www.ninjette.org/forums/showthread.php?t=77150