Maybe be sure they feel like have permission to screw it up when they go for it. Both from you and themselves. Just yesterday I walked my keyboards through the first of a two measure riff. Then I insisted they 'beat the crap out of it' with the understanding that the second half is gonna suck and we're gonna be ok with that for a few days while they work it up but first half should be solid.
I also do playing tests. They play by themselves, recording to one of the six iPod's we own.