It takes about a minute for the APU start cycle to complete. Start the APU when you are a couple minutes from the ramp. Pull up to the gate, set the parking brake, shut down the remaining engine, and, while waiting 5 minutes for the ground power and air to be hooked up, complete all your checklists, put pen in holder, and pack up your stuff. As soon as external air is hooked up, turn off the APU and vamoose. If you don't want to wait for external air, be a big boy and take responsibility for turning off the APU without external air hooked up and drive (or commute) home knowing that you just fooked the next crew with a hot cabin because you didn't want to stick around a few extra minutes to help them out. And further, try not to come on here and b*tch that a previous crew gave you a hot cabin after you know you just did the same thing to a crew last week.

My $.02.