How to get more storage space, per se, for free!
1) Post the item you wish to sell, on the AH for something like 10 gold, and set the auction time to "low".
2) When the item expires, unsold of course (or making you rich IF sold), it will go to your mailbox for 30 days.
3) Retrieve the item anytime in the next 30 days from any mailbox. (Good for herbs or ore/etc). Or, don't collect it and let it just sit there.
The only thing this doesn't work for is Soulbound/Quest items.