Smile and make eye contact with someone in a bar. This is a simple way of showing you have interest. It also indicates you are approachable. You can play coy and look at someone until he looks back, then look away for a few seconds. Look up again and smile at him. Assess his level of interest; see if he smiles back.
Walk up to someone at a sports bar and ask him what his favorite team is. This is an easy icebreaker because you can ask him questions about his team to further the conversation, or, if you know a lot about that team, you can make observations about the team. Smile and make eye contact to show your interest.
Sit next to someone and casually bump her arm at the bar. She will look up at you, which gives you an opening to speak to her. Apologize for bumping into her and offer to buy her a drink. You have just opened the door to further conversation.
Walk past a guy in the bar. Once you have passed him, slowly turn around and make eye contact and smile at him. This gesture signifies your interest. This is a bold move. If he is interested, you will know because he will smile back or get up and speak to you.
Ask someone next to you a question. For instance, ask a girl what the meaning of her tattoo is. Pay attention to her and compliment her choice of tattoo. Alternatively, ask a girl a question about a pair of earrings or clothing she is wearing.
Use humor to flirt. Walk up to someone and make a joke about yourself. Engage her by laughing at your own dance moves. The point is to get her to loosen up and laugh with you and signal her level of interest. If she is cold and doesn't laugh, you can walk away.