Flirting - its a balancing act . . . .

There seem to be two major types of people when it comes to flirting. 
1.  I flirt constantly with anything that moves. 
2.  I never flirt. 
The, trick, my friends, is to be somewhere in the middle. 

Do's
Do flirt a LITTLE with everyone . . . . you never know how much it will help someone's confidence.  And you never know who will take notice, think you seem pretty fun and personable and they really ought to ask you out sometime. 
Do practice.  Not into that guy/girl?  A *little* flirting is harmless.  If you flirt a bit, it gets easier, and he/she gets an ego boost.  win win!

Don't
Don't flirt with married people.  Are you an idiot? 
Don't limit flirting to the people you're interested in . . . because it will be too obvious, too awkward . . . you want flirting to be a personality trait, not an olympic sport. 
Don't take flirting too seriously. 
Don't go too far. 

This brings us to the next point . . . appropriate vs. inappropriate flirting. 
Good flirting is
flattering
happy
fun
ends with normal conversation
appropriately light

Bad flirting is
offensive (sarcasm can get offensive real quick . . . so can teasing about someone's traits.)
constant
pouty
needy
sexy (Yes, sexy.  You'll either look stupid, or disgusting.  Don't lead with your sex appeal . . . lead with your personality.) 
competitive (you can flirt with him/her later, just walk away.)

You've got to keep it light & happy  . . . its like perfume or cologne.  You want everyone to think you smell nice, but not choke anyone.