Besides lightsaber crystals that increase deflection, the Jedi Defense feats, as Evil Q pointed out, also increase deflection. Hand deflection both allows you to deflect blaster bolts without a lightsaber and gives you a bonus when deflecting with a lightsaber.
Beyond that, you'll need to boost your attack bonus. Defense plays no role in deflection and dexterity only applies if you have the Weapon Finesse feat. A successful deflection is based on your attack roll exceeding your opponent's attack roll. Exceeding it by ten or more deflects the bolt back at them.
- Star Admiral