I simply add some chopped pickled Thai chili peppers to fish sauce, maybe some rice vinegar or lime juice, and possibly a touch of sugar.  That's all (aside from washing your hands, which will now likely smell like fish and burn whatever it is you decide to touch).

This goes well on most Thai dishes (like Thai grilled beef salad) when you're looking for an extra punch of heat.  It's sweet, salty, sour, and spicy.  These are the elements of so many Thai dishes already (not to mention other southeast Asian cuisines), so it's a perfect marriage. 

Those pickled chili peppers come in handy when you're making larb as well.  You can get them at most Asian markets, online, and at Han Ah Reum (Hmart)  around the country.