No need to worry too much about storage too much unless you expect to store it for very long periods. Keep it in a sealed container and it will keep for many months without any noticeable loss of potency. I have not personally noticed any obvious difference after leaving it for a couple of weeks exposed to the air, so don’t worry if you have left it out for a short while.
Whilst spice does oxidise (certainly not so much as to be noticeable after a couple of days), it is still active. Some people even say they prefer DMT n-oxide to straight spice.