After the sequence 1X-1M--2M-2NT(asks), I like the replies recommended by Gavin Wolpert:

3C: 3 in M, minimum
3D: 4 in M, balanced
3H: 3 in M, singleton/void in the lower ranking suit possible
3S: 3 in M, singleton/void in the higher ranking suit possible
3NT: 3 in M, maximum
4M: 4 in M, to play

Edit: a couple of days after this posting, Gavin wrote about them in his great-2-read blog but using the cheapest bid over 2M to ask:
http://bridgepro.blogspot.com/2007/04/spiral-raises.html

Also check out his post just below it, for details on how two-way checkback is played by experts.