Sal, are you using an air concentrator attachment on the end of the dryer when you use it? If not, a concentrator will keep the air confined to the section of hair you are working on and prevent it from getting big and frizzy.
Also, have you tried letting your hair air dry before flat ironing it? This is what I do when I want an extra-straight, sleek look. My hair comes out much softer, shinier and silkier than it would if I were to use a blowdryer.