How many users do you have?
So first off, the easy stuff (so I don't need to think very hard). Make sure your running the last Office 2007 service pack. SP2 I think?
Next comes the rules. Excel 2007 will only let 1 person open and edit the file at any given time. You probably know this but I'm just posting it just in case. Be sure your users are training to open the file, save it and close excel. This way the lock should be released.
Next make sure the permissions are setup correctly. Just to keep things simple for now, make sure you give all your users "Full Control" so they can full view, read and modify the file.
After that, you should be fine. I've been working in a 8000 employee company for a long time and we've always shared .docx, .xlsx files with no issues other than the average douche who opens a file, saves it a few times but never closes it.... stupid tool....
There also might be a script you can run on the file server where your sharing these documents where you can clear all connections to the share every night... might help ya?