Eye Contact

Usually the first contact made with a "Potentially Interesting Person" (from this point forward they will be referred to as "PIPs") is eye contact. There are several different types of eye contact.
Here is the "Across the Room" eye contact.
The ideal "Across the Room" eye-contact flirt only makes contact for two or three seconds, and then shyly, must turn away. Usually this will make you smile, which is again a perfect communication - friendly and pleasant. It shows you're happy that he looked back and showed interest in you. Even blushing works in your favor, so don't try to hide it. In these ways, you are using your nervous energy to your advantage. It's important that you let him catch you looking back at him. Subconsciously, this tells him you are interested. If he doesn't catch you looking back at him, he may decide you are not interested. Then he may not be able to get up the nerve to come over and start a conversation with you.

Side note: If he's looking at you when you look back at him, that's a pretty good sign he is interested in you.
If he isn't looking at you, that's not necessarily a sign that he's not interested in you. In a situation where you have the time, this entire sequence of looking, looking away and looking back, should repeat itself a few times before you move forward. This kind of eye-contact flirting is most effective in a restaurant, club, or in line at an amusement park or the DMV. Use it in any situation where you and the PIP are going to be in the same place for a while.