Here's a good link on diacetyl rests: http://www.beerme.com/diacetyl.shtml
Basically you do it when your OG has dropped by half. I do it when the fermentation looks to be slowing down.
As has been covered, there's two schools of thought with lager pitching, that is pitching at room temp and lowering to lager temp, or pitching cool. Usually you trade off some lag time with the latter but some people claim cold pitching improves the final product.
I personally warm pitch to avoid hassle and my lagers turn out fine.