Idling is not the most efficient way to warm up the engine. Nor is it environment-friendly. The best way is to get going applying light load, i.e. no flooring until the engine reaches its nominal temperature.
'08 E92 335i MT / Space Gray / Coral Red / Aluminum / ZSP / ZPP / iPod+USB / Heated seats
+ OEM alarm / 330i SSK / M3 shifter knob / M-Tech steering wheel / BPS Motorsports aluminum pedals + M3 foot rest / RE 12mm spacers / Euro split-view mirrors / Manual rear sunshade