Had a search and didn't see this listed at all.
After the discussion on the "penny pouch" bug being used to pause duration on items, it seemed that few people knew about this.
When an item is in the mailbox its state is put on hold, it can't be modified.
This stops people enchanting things in the inbox, opening containers, and other things.
It also means the wow server can't change the duration remaining on the items, and it will not "decay" over time.
What this means is that you can put your 1day duration items in a mail to your bank alt, leave the mail unopened, and send the mail back. Opening it many days later when you wish to use the item
I personally am using this to store Taffy for more +hit if I need it while levelling, and as a cheat to up hit rating during early raiding at 80.
I see this more as a tip myself, but due to it exploiting the way the mailbox works to bypass a game mechanic I feel it fits here better than any other place. Moderators feel free to move it if I have overlooked a better location.
(first submission, Hopefully more to follow)