To encourage any kind of play, any reward that the players value, or method that gets results by their lights, will work just fine.
One of the reasons that we've had no issue with players being imaginative is that, since before 3E came out, I've had it as standard practice to give out bonus awards for clever or amusing play. If you make another player (or me) roll on the floor or prompt an involuntary, "Wow, that was slick!"--the award is automatic. For 4E, we use Action Points for the rewards, because that fits into the way we play it.
That doesn't mean that every stunt using p. 42 qualifies. Far from it. But a player is more likely to need p. 42 to be clever or amusing, and thus have a shot at such a reward. However, if they can use a power or skill in a clever or amusing way, we don't want to stop that either. And sometimes the player is tired and wants to forgo such efforts without being completely ineffective. That's fine, too. The whole thing almost is self-regulating.
The group should always reward the kind of behavior that they want in some way. The trick is determining the scope of what it is that you really want, not getting hyper-focused on some narrow examples of it. "I wish they would use p. 42 more" isn't often a prime desire, but rather an expression of a larger one. Determine the prime thing, and reward that. P. 42 will take care of itself.
