If you need help picking someone up out in public, we have four foolproof ways to get it done.
1. Be brief and to the point and then move on. In one example, a guy came up to a girl and said that he needed to take care of a sick friend and had to run, but had to compliment her on how attractive she was before he left. She was impressed that he was taking care of a friend and asked him to come back later. People also don't feel intruded upon when you make it brief.
2. Keep fun facts on hand. Useless information comes in handy when you're trying to distract someone from the fact that you're hitting on them.
3. Impress the person's friends first. Since you aren't dating her friends you'll be more comfortable talking to them and it gives you an easy in to talk to the person you're trying to date.
4. Say something sincere. Cheesy flirtations and flattery are pretty easy to see through so why not say something you sincerely mean instead? In fact - why not something that sincerely picks on them? If they're attractive they're used to be told they're beautiful but they aren't used to being told they have lipstick on their teeth.