Ranked 1-20: Smiling
There is no better way to let a person know you are interested in them than to smile often.
Whether the subject of your crush is across a bar, or having a conversation with you, it could even be someone you know, if you smile in her presence, she will naturally feel comfortable with you, and she might realize that you're interested in getting to know her better.
Ranked 21-40: Laugh
As with smiles, laughter breeds warm feelings.
If you find a person who you'd like to flirt with funny, by all means, laugh at his jokes, as this tells the subject that you're on board with his sense of humor. People typically feel comfortable around those who laugh at their jokes, as humor is a personal characteristic. Laughing at your flirting target's jokes tells him that you could be interested; particularly useful if he's also interested in flirting with you.
Ranked 41-60: Ask Away
People are naturally drawn into conversations with people who inquire about their lives.
If you're interested in someone enough to make him a flirting subject, you most likely want to know him better, to determine for yourself whether or not he is worth flirting with. Ask the subject anything that feels appropriate in a particular situation. Where does he work? Does he enjoy his career? What does he aspire to, if anything? A good rule of thumb is to hone in on questions that you yourself would most value being asked. Whatever the question, if you think it, don't be afraid to ask.
Ranked 61-80: Touch
Personal space can be a tricky thing to honor, and if your flirting subject is shy, you might have to read the situation.
But if you and your crush are in an informal environment, say at a work function, or at a dinner with friends, gently touching a person's arm, when you find something funny, can be effective in giving that person a signal that you're flirting, and interested in possibly going on an individual date. Women, bear in mind that you have even more control in this situation, as men can be careful not to touch a lady unless she reaches out first.
Ranked 80-101: Eye Contact
Don't underestimate the power of facial expressions during flirting.
Looking someone in the eye is an intimate practice, which is why many people have a difficult time with it. If you want to flirt with someone, you must show her you're not afraid to be intimate with her. Similarly, if your subject is not comfortable looking you in the eye, it may be a signal that you'll have a hard time getting to know her further down the road. If so, you might be best to find a new flirting subject. To be sure, you should enjoy the process while you're out there; flirting heeds the best results when it's fun.