You use the formula:
LST = K + 1.0027379 x t + 0.06570982 x D - Longitude/15.where, t, is the Universal Time in decimal hours, D ,is the number of days since December 31 of the previous year, and, Longitude, is the west longitude of your location. The constant, k, has to be looked up in the U.S. Nautical Almanac and is the Sidereal Time at 0h Universal Time for December 31. This quantity cannot be easily derived with any accuracy and requires a detailed dynamical model of the Earth-Moon-Sun system. The table below gives recent values in decimal hours:
Year.............K...... 1994 17.371435 1995 17.387349 1996 17.403263 1997 17.353468 1998 17.369382 1999 17.385297 2000 17.401211 ........................