ATMA is a very handy program for Windows (only?). It allows for you to open d2s files (saved characters) and simply move stuff on and off characters. Google it, and you're sure to find a download somewhere.
ATMA has a lot of other interesting features as well, like the very cool drop calculator, which will (when used correctly) display odds for a monster to drop what items, or what monster is most likely to drop a spesific item, all taking MF and number of players in the game into account.
Muling can also be done by using two computers in LAN (or with a trusted friend on open) but atma is MUCH safer, as it will save a backup of your characters, in case you make a mistake.
Also, you can create a "Stash" with ATMA and end up having an infinite amount of space to store anything and everything. I created several stashes, each with different names (ie. Uniques, Rares, etc.). It makes it easier than saving each item as a seperate file. You can view the properties in ATMA and drag and drop them.
On the pulldown menu Utility, click New Stash. Name the stash as you want it (ie. Uniques, Rings, Charms, Set, etc). Check the path so you know where its going. Now, to open the stash, you'd do it as you open one of your characters, except it will now appear on the D2X tab on the Character/Item View. Just drag and drop stuff into it and you'll understand.