Our company guidance includes:
Closing the blinds, put up cockpit shades, open all the overhead air vents.
Close the overhead lockers to reduce effective cabin volume to be cooled.
Use the APU instead of low pressure conditioned ground air (we're not permitted to run packs, recirc fans, and LP ground air together).
If needed we can put a LP ground air hose through the service door into the cabin. This bypasses the limitation about running packs with LP air. If the APU is MELd we can have both the normal LP air connection and the service door method.