First off, mad props for making the effort to talk to girls.
In general, shopping stores are a tough place for a beginner to hone his skills because he's often unsure whether or not the cute sales girl is being nice because she's interested in him or just trying to sell him something.
Supertrooper, the best thing to do with every girl is look your best, make casual small talk and close by at least asking for the home phone number. Don't concern yourself too much with IL. Yet at the same time, don't pester a girl who is obviously not interested in you either. Anything other than a yes is a no.
Run a search on "opening lines" so you'll get a few ideas. In fact here's one:
A few months ago I was in a long check-out line at a local Wal-Mart. By the time I reached the counter, the cute sales girl was noticably tired. Seizing the oppurtunity to hone my skills, I smiled and asked, "Where would you rather be right now?"
Her reaction was priceless! "Are you kidding? I'd be out getting my hair and nails done, I'd be at the park with my kids, then I'd go home and blah, blah, blah...What would YOU be doing?" She enjoyed the convo so such that she held up the line 5 mintues just to talk to me. I didn't number close ,though (I'm engaged.)
Point is, my opening line of "Where would you rather be right now?" let her know immediately that I empathized with what she was experiencing at that time. It set me apart from the other customers. Again run a search to find other ideas.
Malls and shopping stores are good for beginners to practice eye contact and saying "hi" but a little bit on the difficult side for number closing. If it seems like you're not getting any success at all, try environments with a lower level of difficulty like classrooms or house parties.
Again don't overanalyze things. Just maintain a clean and neat appearance and put your best foot forward. Use common sense, stick to the basics and you'll develop just fine.