I reached the maximum capacity on Zotero's server, so I got a box.net free account (good for 5GB!). I was going to try to use my Google Document storage, but this is easier. I think Dropbox is an alternative, but I want to keep my free storage there available for other items.
Go to Zotero's advanced preferences and sync via WebDAV.
Enter the following address https://www.box.net/dav/zotero.
Enter your box.net username and password. Click "Verify Server." It will ask you to create a new folder--say yes. Then click "Verify Server" again, and the sync process will begin.