Adding extra sulfur to the soil and the foliage of Hot Pepper plants will help the plant build compounds that lead to a hotter pepper. Dissolved Epson salts sprayed on the soil and foliage of a hot pepper plant twice a season should be plenty of addition sulfur.
Proximity to Mild Peppers
Most pepper plants will freely cross-pollinate with each other. The results of a mild pepper (such as a bell pepper) crossing with a hot pepper often could be a more mild hot pepper. The resulting seeds from these hybrid peppers will be quite variable from hot to mild peppers as well.
Keeping mild and hot peppers, and even different heat levels of hot peppers away from each other helps eliminate the cross-pollination of peppers.
While all of the above techniques will help to produce hotter hot peppers, environmental factors also affect hot peppers. Hot temperatures are needed by many hot peppers but control of the weather is not possible, yet at least, so by controlling the amount of water a plant gets, not allowing excess amounts of nitrogen, adding sulfur and keeping mild peppers a good distance away hot peppers will be encouraged to produces the hottest peppers that they can.