alt Hacker NewsGiving them slightly acidic water off-rig and normal water while playing is another option - not the best either. I opted for sugar in the end, because they didn't spend much time on the setup; but this should be reconsidered if they on-rig for hours daily.
The trick is to save the treats till the end. They can learn to do a lot if they know they'll be rewarded later.
Check out Shadow The Rat on YouTube; she has a whole series on training them.