A few things. You could try moving the source of the WiFi (your router, or gateway) more towards the center of the building to help get an even coverage, or for even better coverage set up a 2nd "wireless access point" with an Ethernet connection to the main router/gateway; you would want the 2nd WAP to be in the general vicinity of the area you're trying to reach. There are "WiFi repeater" devices commercially available, but my experiences with those have not been ideal; I certainly wouldn't recommend them in a business-critical setting.
Also, be aware that Comcast recently launched a managed WiFi service called "Business WiFi Pro". It costs extra per month, but they give you some professional hardware and do all installation/maintenance for you.