I have a list in my head of some quick easy openers that work consistently.
Most of these are of the L.I.O. (Low Investment Opener) style.
Clothing
My favorite one so far has been commenting on clothing, especially hats and jackets.
If I see some girl with a really cool hat or a furry jacket, I'm like "I like your jacket".
Or "That looks nice on you.".
Usually, you get a smile and "thanks". So then I'll pause and sit just out of reach and if she looks at me, I re-open.
Real simple.
Waves
I do this silly wave that I learned from my buddy Manny called the Manny Wave.
Its a very fast wave coupled with a silly grin.
Works great.
Again, let the sexual tension build up, then go in.
Distracted Lockin
Another friend of mine has also experimented with this one and confirmed it does work. It causes girls to give you IOI's.
You just post up somewhere and put the girl in your peripheral vision. Use your more dominant eye to do this.
Then just sit back and wait. Keep her in your peripheral vision and watch her.
She will see your profile of the side of your face but subconsiously it will register that you're looking at her.
Watch the sexual tension build up.
Its important to pick your venues for this. Alot of distraction may cause it to evaporate if there's alot going on but it generally works most of the time.
As the sexual tension builds up, you can open with anything. "What's up" is fine.