Belch really loud and deep (it's one of my sexual fetishes).
Otherwise, he could express some positive level of confidence in himself and a degree of self-esteem that doesn't leave him a neurotic drama queen. He should dress and groom well (whether formal or casual). He should also be willing to share of himself in a conversation (as I am a little too willing to blurt out too much info about myself).
That's just basic level of impressiveness though. To really impress me, he would reveal that he has initiative and drive to seek out and do the things he wants in life. He must show a courage that I myself desire to achieve; in other words, he must inspire me by virtue of his own being.