Can I prevent hot pepper sauce losing heat?
The heat of your condiments isn't actually being lost. The condiments are marrying, meaning the heat becomes more homogenously distributed through the condiment. This means you don't have bits and pieces that have as high a spike in heat than the rest of the salsa, and therefore the condiment is more evenly hot (thought apparently cooler to the taster) throughout.

Pepper or chile sauces use vinegar and oil to retain and even accent their perceived heat. Neither actually increases it though. Vinegar helps clean the palate like strawberries for champagne, or ginger for sushi. It cleans the tongue so the heat can get to it better. Oil helps hold the heat to the tonque and other mouth parts making it seem hotter because it doesn't wash away as easily. 

Keep in mind though that pico de gallo is really meant to be made and served fresh. You don't want to give it time to macerate and marry. You want to bite into and taste the individual components of this condiment. Leaving Pico to sit overnight in my opinion really turns it more into a chuncky but watery Salsa.