Before storing them, you should also "retension" them. That is, fully fast forward to the end of the tape, and rewind to the beginning. That will re-spool the tape in an orderly fashion, and avoid curling of the tape due to loose packing.
Retensioning was a common practice in the older days of computer tape backup, especially with 2-spool designs, which a cassette is. I agree with Goinbroke, though, any magnetic media will steadily degrade over time, especially well-used tapes, as much of the magnetic coating on the tape will have already worn off through contact with the tape heads during playback.
Though I'm not an expert, I would make a guess that any storage container you use should not be completely airtight, as trapped humidity could cause damage. To achieve better humidity control, you could use silica dessicant packs (the "do not eat" packets that come in the shipping cartons of many electronics - these absorb humidity)... these might be a good idea to throw in the storage container as well.
Retensioning was a common practice in the older days of computer tape backup, especially with 2-spool designs, which a cassette is. I agree with Goinbroke, though, any magnetic media will steadily degrade over time, especially well-used tapes, as much of the magnetic coating on the tape will have already worn off through contact with the tape heads during playback.
Though I'm not an expert, I would make a guess that any storage container you use should not be completely airtight, as trapped humidity could cause damage. To achieve better humidity control, you could use silica dessicant packs (the "do not eat" packets that come in the shipping cartons of many electronics - these absorb humidity)... these might be a good idea to throw in the storage container as well.