After primary fermentation is complete (typically around 50degF), and an optional diacetyl rest is completed, lagering should commence. Many will tell you to slowly lower the temps around 1-2 degrees/day (so as not to shock and slow the yeast in their metabolism) until you reach your lagering temp. Once I perform a diacetyl rest I typically cold crash the beer over the course of several days, then rack to a keg for lagering. I have not had any issues doing this and have had good results with my lager brewing in this fashion. The colder the better for lagering. I usually lager around 32-34 deg F.
When I rack to the keg I purge to seat the lid with CO2 and to clear the headspace of oxygen (or at least as much as feasible). I usually let the beer lager for several weeks (depending on the gravity and style) before carbing it up. My main reason for this is I feel that dissolved carbon dioxide in the beer will inhibit the yeast due to the added pressure exerted on it. Maybe I just feel bad for them....
