Smile with both your mouth and your eyes. When you smile and maintain eye contact with a man or woman, it sends a signal that you're open to him and like what he says and how he looks. Also, tilting your head while you do this gives a woman the impression on a purely biological level that you really are interested. Refrain from crossing your arms out in front of you and always turn the front of your body to face his direction, even when you're across the room (See Reference 1, 2).
Touch him or her with small little caresses using your finger tips on his arm or shoulder. You want to remain a little subtle so that the guy can get used to the idea of you possibly liking him. According to GirlsAskGuys.com, men tend to prefer it when women do not come across too strongly in their actions, as biology dictates that men prefer to chase women as opposed to women pursuing men. For men, if the woman is using the same body language that you chose to use earlier, you can assume that small touches won't upset her.
Let her know that you're interested in her life and personality by asking questions. Ask him about his family, his favorite movies and music, as well as his aspirations. Keep complete attention on her as you talk to her, maintaining that eye contact, and including the small touches. Don't forget to be a little flirty in the conversation by lightly teasing her and letting her tease you.
Talk to him about your feelings when you gain some confidence that he may feel the same in return for you. You don't have to pour your heart out and tell her the exact moment you knew when she was the one for you, or the way her eyes sparkle when she laughs. Just keep your emotions on the down low when you tell him what's on your mind. You can say, simply, "Hey, I like spending time with you" or "You're so much fun to be around," or flat-out saying "I like you." For women, if you feel extremely confident about his feelings and feel that he likes you the same way you like him, you may want to let him tell you to keep his interest, as GirlsAskGuys.com suggests. A woman wants to be the pursued, not the pursuer. If he never expresses his feelings, chances are that he doesn't feel the same way you do.