This is an obsession of mine. (being of the anal-compulsive syndrome) I may have come up with a partial answer. Here's the theory: what dries out any product is exposure to air, right? Well, we can't totally 100% reseal the tube, but can we keep air away from the opened tube? YES ! Submerge the tube in an appropriate sized container, such as a cigar tube, making sure the opened end is under water! Remember to squeeze the air out of the tube.
Logically that should work, or at least drastically slow down the drying-out process. And I don't care if I could buy the stuff for 30 cents a tube. We are talking principle here!! (see above noted syndrome)
Tony Ryan