While you might be spending your days dawdling in Dragon Soul or messing around with dungeons, the subject of an upcoming expansion is actually one you might want to turn your attention to. A new expansion isn't just more levels to play -- it's more items to collect, a new market of trade goods to delve into, and a host of game changes you may want to prepare for. Prepare for? Yes, absolutely. After playing through three separate expansions myself, I can tell you with certainty that preparing for an expansion before it's released will save you a lot of headaches in the long run.
That bank alt is filled with all kinds of low-level trade goods and supplies that are doing nothing but collecting dust. I have a huge stockpile of blue-level gems from this expansion, as well as some old gems from Wrath. My problem in Cataclysm, as far as the bank alt is concerned, is that I didn't do my normal spring cleaning of the bank alt before the new expansion launched -- so all of the old stuff continued getting older, and my bank alt's storage continued filling with more stuff.
Whether you have a bank alt or carry everything on your main, cleaning out all unnecessary items before a new expansion launches is a really good idea. First, it gives you more space for the current level of trade goods, which is the stuff most people really want to purchase. Second, it puts a little extra gold in your greedy little hands -- and who doesn't need more gold, particularly when there may be Something Expensive on the horizon?
Old-world materials used for crafting professions like cloth, leather or gems can either be sold straight away or saved for the inevitable glut of leveling pandaren who want to quickly speed through profession leveling. However, if you're going to save it, don't forget to list it when the expansion actually goes live. The longer you wait, the longer those items will be collecting dust in your bags. Everything in your bank is something that could potentially make you gold.
The two single most valuable things you can take into a new expansion are gold to spend and space to put all that stuff you spend your gold on. Cleaning out your bank and making room well before the expansion starts guarantees you'll have both -- and you won't be blindsided with a lack of either on launch day.