If your firm uses off-site computer storage (i.e., "cloud storage") for its computer files, this makes working from an alternate site much easier. First, your files are accessible from any location with Internet access. Second, you do not have to worry about the catastrophe at your office space also destroying your client files and data. Of course, you will have to ensure that your staff has computers to access the files. This can be accomplished either by renting computers during the recovery effort, or making sure that everyone has a computer at home. Laptops that are used solely for work purposes (in the office, as well as for work at home) are a good idea.
The U.S. Small Business Administration's Prepare My Business site has a lot of useful information to help in preparing for business continuity operations. Particularly useful for alternative work site preparation is their Recovery Overview checklist.