There are at least 4 different defenses that I know of that are affective.
1st.Hands in close...some call it the "peek-a-boo"...Just watch Winky Wright
2nd.This defense kind of baits your opponent.You hold you jab hand low which allows a counter hook or you slip under their hook and block a jab with your other hand.-I've seen roy jones do it a couple of times.
3rd.Rolling and Parrying with your shoulder-Watch Floyd Mayweather.
4th.You hold both hands low so that your body is covered and you rely on side to side head movement to dodge head shots-A lot of the old timers did this.