Your question, "how to store a print head that is being used" doesn't sound like it could have an answer. Doesn't the word "store" mean it's not being used? I really don't know what you are asking.
EDIT: I composed my response while embguy was also responding. He, of course, gave an excellent answer to how you should store a printer that is not being used and what you should do but it sounded to me like you wanted an answer that did not require removing the print head from the printer.
One thing I would add to embguy's response: you should seal the outlet port of your cartridges when they are not in your printer. You can do this with the original orange seals they came with using rubber bands to hold them in place or you can stretch a piece of vinyl tape across the outlet port. Also, if you use the German method of refill, you should seal that hole, too. You should not seal the air vent hole on the top of the cartridge. It is the proper way for the cartridge to breathe in and out as the ambient air pressure goes through its daily fluctuations.