Being unable to control themselves physically
When they see you for the first time in awhile, they won't be able to help but run up to you and spring off the ground to throw their arms around you. The guy must then either spin around with her still holding on, or lean back to give her body extra distance from the ground. If they don't spring onto you, they will still stop short of you, push themselves up onto their tip-toes, and lean forward to hug you, so that holding onto your body is the only way of keeping their balance.
When making a point, they rest their hands on your shoulder and arm, or playfully hit you -- and not in that calculated way where it seems like they're aiming to seduce you, but because they just can't keep their hands to themselves. The look on their face is not "Oh, you...." but one of angry surprise or mock-disgust.
To get a boy's attention, they tap or stroke the chair in front of him at a table as they walk by, fluff their hair, tap or hold onto or stroke the jamb of a door as they leave a room that he's in, tug lightly and repeatedly on his sleeve when he's not looking, and bend over to rest their weight on a chair or table in front of him while arching their back. None of these is done is a slinky or seductive way, but in a perky and anxious way.
The common factor here is that involuntary behaviors like these signal that she has no control over her emotions, which reassures the guy that she can't just turn off her interest in him like a switch. By contrast, moves that appear more seductive signal that she's only interested in a quick fling and won't hang around -- which may be great if he's just looking to get laid, but not if he's judging potential long-term girlfriends.
How to preserve this charm:
This is really easy: just do the above behaviors. You may have to practice a bit, since you're probably rusty, but just remember that you should project an air of involuntary nervousness, like you have butterflies in your stomach. The hug move where you stand on your toes and lean into and onto him for support is all-purpose, since there don't have to be tables or chairs or door jambs nearby to caress as you walk by. Try psyching yourself up and walking fast before reaching him, since there should be a little spring in your step right before going for the hug. End it by opening your eyes wide and putting on a big genuine smile (where the muscle around your eyes squeezes to form crow's feet), and he will take notice of how different and refreshing it feels to hug you compared to other women.
Method actors try to recall past personal experiences where they felt the way they need to be for a character, and this should help here as well. While men have better reasoning abilities, women have better memories, so this is easier for you all than for us. Try remembering a specific guy you had a huge crush on in high school or college, and how nervous you were right before you went up to hug him for the first time. Or perhaps how you came back from summer camp or summer vacation and saw a guy friend for the first time in months, ran up to him, and jumped into his arms. Really, anything like this that you have a vivid memory of.