Last week, the Electronic Privacy Information Center (EPIC) made national headlines (including this one in The New York Times) with the release of the 92 pages it received in response to a FOIA requesting information on how people make it onto the government's terrorist watch list. While interesting in and of itself in terms of content, the EPIC FOIA also highlights some of the ways federal agencies (especially law enforcement agencies) deal with FOIA requests.
First, there is the (expected) delay. In almost all instances, FOIA requesters should expect a substantial delay beyond the statutory limit for making a response (20 working days with an extension available plus other statutory reasons to stop the clock; known as "tolling.") Anticipating this, EPIC requested expedited processing of its request. Expedited processing is available to certain classes of requesters. To see if you qualify for expedited processing, reference this page.
Then there are the claimed exemptions. Follow this link to the FBI response to see how the Agency employed exemptions and how it documented the exemptions it claimed. The response also annotates each exemption.
Finally, you should read EPIC's original request. It is well researched and drafted.