I have swang! I try to clarify every time I straighten so I don't have alot of product build up. After that and my Deep Conditioner I use a light leave in conditioner and then Air Dry! For me the key to swang an no swang is how heavy I am with products. The last product I use on my hair is Heat Protectant.
IMHO, Because we are trying to be cautious with our hair during the straightening process we tend to use a tad toooo much product just scale back and you will have swang.
Another secret (thats probably not a secret, lol) I have is after flat ironing instead of bumping the ends to get a curl, I make these really BIG grand-ma curls (yes, this is what I call them) then I wrap my hair. This allows the hair to be fluffy when I unwrap rather than stiff!