"Tetuso Turunku Rumu" comes recommended. They have a storage facility a 7 minute walk from Toyoso Statin on the Yurakucho line (in the Kiba area). You can load all your stuff into a container which is about 8 cubic meters in size (2x2x2meters). The storage is around Y12,000 per month. Daily access fee is about Y3000 or Y3500(?). They are helpful (if you or a friend can communicate in Japanese) in small ways. For example for my friend they brought his cube out into the parking lot and let him unload, sort, toss and/or re-pack in their parking lot for about 4 days, and in the end even took about 12 boxes down to the post office for him (this was a free favor). Here is the info as best as I can glean from their card:
tel: 03-3644-3361
fax: 03-3644-0166
toll free: 0120-28-3361
web site: http://www.tetuso.co.jp

And it looks like they have a location in Yokohama as well? (Take a look at the web site even if you can't read Japanese because there are a lot of photos that tell a lot.)