Show me the way!
You need to stop it with the cheesy one liners. She'll have heard them all a thousand times before if she's even remotely pretty.
Next time you see her, make a mental note of the following things:
Hair colour: As in from root to tip. Does it graduate or is it the same colour all the way through. If it graduates, she'll dye it. That graduation is your benchmark for next time. If she's a natural, just take note of the style. Then you have a way in next time she does something different with it.
Eyes: As in eyeshadow colour. If she's got quite expressive eyes chances are she'll like to play with colour, blending and using quite bold shades. Usually a girl will go with either eyeliner and mascara with neutral eyeshadow or brighter eyeshadow with minimal eyeliner. Take notice of all these things.
Then, next time you see her, notice what's different. Mention it. Girls love anyone noticing anything about their hair and eyes. If she's interested, she'll respond
