Here's a great solution I stumbled upon from an old comic book collection I had—comic book longboxes:
If there's a comic store in your area they'll usually have these cardboard boxes in stock at a good price. They're just the right width for a 45 and I'm guessing you can fit 250-300 per box.
While I bought mine at a local store, here's the type of box I'm talking about (I've never used this store):
http://www.tcbulk.com/scripts/Long_Comic_Book_Storage_Box-Bundle_of_10_boxes-ur2-149.asp
Ten boxes for $25 doesn't look like a bad deal, but ten boxes might be overkill—if you can find a local store you can usually buy one box for a couple of dollars.
Because these boxes were made to hold comic books, there's a little extra space between the top of the 45 and the box, and I've used the space to help categorize my records by label:
The boxes stack well, keep your vinyl clean, and if you make your own labels, help you stay organized. (BTW, these come with lids, too)
