Drummer hit my sm57....how can I fix the rattle now?

Sphincter micing is an affecting technique. As we can see here:

AStech1.jpg
 
get a rubber o-ring to put around the gap between the cap and mic. If you can't find on that fits exactly, get 2 smaller ones. Make sure they are nice and snug. All my new 57's rattle and ended up putting them on all my 57's even the old ones for good measure. I tried tape but it seemed to kill some low end response.

by covering that opening on 57 you change the pickup pattern. The 57 will lose alot of it's off-axis rejection. that little opening allows the mic to use phase cancellation to give the 57 it's directionality. If you don't believe me, take a 57, hold it by the bottom and talk as you pivot the mic - you should hear good rejection on the sides. Now, take you hand and cover that gap, and do the same thing, it will pick up more sound than before when it's turned sideways.

Edit: i see everybody's x beat me to it.
 
I feel horrible for making this analogy... but it works.

Shure_SM57_LC_Professional_Microphone-resized200.jpg



If you imagine the 57 like a penis, they're putting the O-ring just under the head, before it flanges out.



...it sounded a lot more scientific in my mind, at least.