I use a clean rag, lightly sprayed with WD-40, to wrap my gage blocks in.  Then I put them into a zip lock freezer bag.  So far that seems to work pretty well.

Fran,
I thought they were nickle plated so they won't rust. Mine have never rusted after all of these years. I just wipe off the couplant with a soft rag and put them back away, so far they haven't rusted.(wait, the couplant I use does have some anti rust stuff in it)Exosen 30

Chet's idea should work fine or a light coat of vaseline smeared on them...

DoAll makes a good gage block preservative they are out of Chicago