A good PU line must appear natural, so the girl thinks you approached here spontaeously. It must never appear like a canned line. Although that shouldn't prevent you from doing a little homework before you start flirting with her. Have some canned lines in stock (the internet has loads of em) - but always try to adapt them to the particular situation. For example take the classic "you look like a girl who knows about X, can I ask you something? OK ... . {insert question about X here}. Ask about which are the tastiest apples to buy in the supermarket, or ask her for an opinion about which if 2 shirts to pick in a clothing store etc.
A jackpot pickup line makes you appear irresistably interesting, and different then all the other guys who hit on her. For this reason you may compliment an average looking woman, but never compliment a good looking woman straight off the bat. The last 6 guys did that - you are going to do something different - like do a joke at her expense - it works because it it shows her that (a) you have a sense of humor and (b) you are not afraid of her. For example if a girl fumbles with merchandise at the store you could simply smile like - ooh you're so cute and helplessly goofy and simply say - need any help with that?, I won't tell the manager if you promise to be nice and not hurt my tender feelings (with a wink) ... Think about what this does it practically insures you against flat out rejection. She might not go to bed with u that night but she just cannot reject you cold - you ruled it out - get it?.
Remember body language , posture and tone of voice are actually more important than the actual words.
And finally - a pickup line is a girls unconscious way of determining how funny, smart and confident you are. So originality (or the appearance of) goes a long way. So just practice - without fear of the consequences (there are always a bazillion other girls to start afresh with). If you think of an original line but the girl slipped past and you couldn't deliver it, thats OK, its good practice that you though of it, next time you'll get her. With time and persistence and belief in your self worth you will become a natural, your brain will effortlessly summon up something witty to say in any situation.
