I ferment my lagers at 50 degrees until they reach 75% of their expected final gravity (about 10 days), and then taste for diacetyl. If I'm using a yeast that produces very little diacetyl, and I don't taste any, I go ahead and let fermentation finish at 50 degrees (another few days). If I am using a yeast that is noted for diacetyl, and I taste a little slickness in the sample, I raise the temperature to 65-68 degrees for 24 to 48 hours for the diacetyl rest. Then, I rack the beer into a carboy, and begin the lagering process. I start at 50 degrees, and then drop the temperature 5 degrees per day until I am at 34 degrees. I keep it at 34 degrees for 6-12 weeks (depending on what I'm making and how "big" of a beer it is) and then bottle.
I pitch my yeast cold, so I don't get a ton of diacetyl even with big diacetyl producers, so I skip the diacetyl rest about 75% of the time.
Here's some great info on lagers:
How to Brew - By John Palmer - What is Different for Brewing Lager Beer?
