Stress before picking, in any way imaginable.

Djole

Fortunately for me I live in MD so high humidity attributes to pepper heat and the use of Alaska fish fert 5-1-1 and mild to moderate stress. The variety of course helps.

Mark

Yup, stressed peppers crank up the heat in their pods. Generally this is why the peppers I pick in the middle of the summer are the hottest, simply because it is *SO* hot here (typically 100F+) and the soil dries out so quickly that the plants are always wilting fairly badly from the heat (aka they are stressed).

Thats the safest way I know of to stress peppers in order to make them hotter, withhold water for longer than you normally would.

I didn't know high humidity attributes to pepper heat. Humidity is
something we have a lot of here in summer.

Lack of water. Make sure they get good sun levels. Really, it is genetics and climate. My plants end up wilting between waters which is a good sign to water again. It stops me from ever over watering. Of course I never let them wilt too long.