Perhaps dragons have a gland or something which produces a flammable liquid on will (a liquid makes for better aim than gas). And maybe it has 2 dry membranes under its tongue which can rub together and make a spark. So it expels the liquid cleanly through it's mouth, without letting it touch the walls of it's tongue/cheek/etc. and simultaneously creates the spark to ignite the liquid right as it leaves its mouth. So the liquid isn't on fire when it's still in the dragon's mouth, it just starts burning a microsecond outside it's lips?