How would you express love and affection for a teen of the opposite sex?

A hug and a kiss seems to work pretty well for most people I have seen. Pretty much like what you would do for an adult, too.

Nice try, Chris Hansen.

Follow his or her lead. For some, a hand-shake is almost too much. Others like hugs and pecks on the cheek, whether or not you remember them as infants.

I hug my step-sons and nephews, because we are a family of huggers.

Writing a large check doesn’t hurt either.

Similar to @gailcalled, my nephew will accept a hug or a slap on the back, but really just wants my money.

Conversely, my niece is all about the hugs and asks for nothing.

Who do you suppose we do more for?

A hug will suffice for me anyway. Teenagers can be touchy.

Ask, “Can I give you a hug?” Then do. You’ll know by the reaction if it was appropriate or not.

Give the little bastard a noogie while choking them out. Is there any other way to do it?

I found some our best communication happened in the car. They may not have been willing to hug me, but they poured their hearts out to me when they knew I had to keep my eyes on the road.

Hug ‘em, a solid affectionate hug, but not so much to cause misunderstanding, as @wundayatta observes: pretty much as I would an adult of opposite gender.
A kiss on the cheek usually seems to be welcome.
That’s for youngsters I know.

If I don’t really know the person well, a respectful handshake is fine.