There's been a lot of discussion here lately on this topic. You might try a search on mud flap.
With what you have now the consensus was to ensure the existing flap is at least 3-4" off the ground when at ride height to avoid having it kick up debris. Beyond that the Protect-A-Tow type devices that stretch a net between the two vehicles seemed effective. Bras and the like got mixed reviews since they can trap moisture and grit and cause their own problems.
Read through the previous threads for more.