Digital BW, The Print

I recently switched to BO printing with eboni ink on my 2200 but I 
occasionally do color printing with the Epson Photo Black. I find 
that if I leave the eboni cartridge out for a few days, it gets 
pretty dried up (or at least thats what I think is happening). It 
just took me 5 cleaning cycles to get an almost brand-new cartridge 
working again and another time I ended up trashing a cartridge that 
I believe was at least 1/2 full. Is this normal, or are there any 
suggestions on storing the cartridge when not in use? Now I simply 
lean it slightly sideways, chip side up against a box that I keep 
near my printer. I recall reading something once about putting tape 
over one of the holes but now I can't find that thread and I don't 
remember the specifics. Thanks for your help!