Digital BW, The Print

Now that I have a B&W set up I like with the UT7 carts, I still find 
that I want to do a little color.  Any clever tricks for sealing up 
carts when they are out of the printer?  I expect that for each 
changeover, the carts may be sitting around for a couple weeks.  I 
have generally wrapped them tightly in plastic wrap, which is kind 
of annoying.  I don't have any good measure on how much ink I am 
losing to evaporation (and, thus, how much more intense the 
remaining ink is getting).