The algorithm calls for computation of 16 sub-keys; these taken collectively are the so-called key schedule.
Joe Kilian and Phillip Rogaway wrote a paper describing DESX and its added protection against exhaustive key searches.
Ron Rivest has written a paper describing a means of testing the correctness of a DES implementation in software.
Eli Biham has a new software implementation of DES that is perhaps the fastest known for 64-bit machines.
This is only the tip of the iceberg; many more relevant papers are available on the Internet.