To do this you will need a slow connection. Lagg is your friend
1. get backpacks and fill them to the max ( also fill them with the small bags full of stuff the ones that can go inside big backpacks.
2. Once you have over 100 items inside bags goto the bank
3. wait for the most lagg and quickly put all the bags into the bank
before it can tell you its full.
I have been able to do this 3 out of 4 times. This works the best on dialup connections. or i heard you can cause high lagg by unplugging your ethernet cable from your comptuer real quick and plugging it back in before it disconnects you from the game.
I hope this helps someone. I had alot of stuff with nowhere to put it untill i was at my bank with lots of lagg and was able to put over the max of items in it.