First contact:
Girls:
Give him that friendly smile and look away, or walk past him and make eye contact.
Guys:
Look for subconscious signs of interest -- frequent blushes, wetting or biting her lips, playing with her hair or jewelry.
Confirm her interest by smiling.
Approaching her will be easier if she smiles back or offers you encouragement after you've sent her a note or drink. Talk to her friends if she's with a group -- she'll be drawn towards that conversation.
First conversation:
Guys:
A simple introduction, as an opening line, is most effective. Talk casually about everyday things -- except work. Let her ask.
Don't push your luck -- if she doesn't smile or her voice is neutral, move on.
Never call her sweetheart or other pet name -- or try to impress her.
Girls:
Show you appreciate his attention by casually touching his hand.
Maintain eye contact -- your dilated pupils and frequent blinking will convey your attraction.
Initiating a first date:
Girls:
Lightly touching his leg or shoulder, matching the tone and pitch of his voice, or laughing with him, are all clear signals that he can safely ask for a telephone number or, even, a date.
Guys:
Your reward could be the ultimate flirtation -- she may smile and tilt her head slightly, or even invite a goodnight kiss.