The 737-800s have had a lot of trim air faults in the early days. There is a Boeing FCOM Bulletin titled Nuisance Zone Temp Light Illuminations on 737-800 Airplanes. The procedure is as follows:
After landing, prior to first power transfer:
Trim Air Switch
.. OFF
Air Conditioning System
____Pack(s), BLEEDS ON
Prior to takeoff, after last power transfer:
Trim Air Switch
.. ON