I don't understand the need to flirt with a person you think only as a friend.
Guys, you never flirt with your male friends, don't you?
Neither girls flirt with their female friends, right?
Why the hell you have need to do it with your opposite sex friends?!?
Some say for fun! There is nothing funny about playing with someones feelings, since emotions can get mixed up really fast. One warm look, or touch (from either side).....and all of a sudden..... you have a problem of "friendzoning" and broken friendships and million questions on this site and all over the internet: "Does he like me as more than a friend?"
Don't flirt with people you don't have any other interest than just being good friends with. I know it can be fun, but think that there is a great chance that someone catch feelings and eventually get hurt and heartbroken. Even if you clearly tell them you don't see them in any other way, your behavior is saying different and our brain is programed to observe wide picture (face expressions, body language....) and not just listen to the words. We react to touch many times and we are not even sure why?
We are so very different, males and females, and not to say every person to itself, so you never know how a person is going to react. So to avoid misunderstandings where ever we can, good start is not to flirt with people you have no sexual or romantic interes in. Always think twice what you're about to do or say.
Have a happy friendships with your opposite sex friends since they enrich our lifes.