Fly the Dash 8 "classic". We don't have an APU and most planes does not have window shades... What we is to put up reflective screens in the cockpit and open the hatch to the vent position. We can also remove the hatch fully, but it can be PITA to get back in place again. Also, if we are in the airplane, open the door to baggage compartment from the cabin before starting bording to get some airflow through the cabin. Other than that, turn on bleed air as soon as we have engine running. Luckily, there are not many days where this a major problem though. Cold on the other hand is more of an issue. Most airplanes have built in ground heaters that will run as long as we have ext pwr connected. In cases where they don't work and there is no external heaters available, an option is to start the engines to get some heat in the aircraft and then shut them down again just before boarding. And also, surprisingly for an aircraft built in Canada, if you flying long distances (3+ hours) and with a light load, it tends to get quite cold in the winter time.