Did this yesterday, worked every single time!! Approached about 4 sets of girls because of it.
Go to the park with a frisbee, can't be a ball gotta be a frisbee (preferably one of those soft rings), and your wing. Don't be playing with the frisbee, just be next to each other talking, it's more obvious this way (we were just laying in the sun drinking beer). When you see a group of girls one of you blatently throw the frisbee towards them so it lands in their general facinity, the other one of you call out "excuse me (wait for them to look), can you pass me my frisbee please". Immediate giggles on their behalf as they can see there's no way you could have been playing frisbee just chilling on the grass. One of them returns the frisbee. Comment on her throwing "you throw like a girl", or "nice throw, you play frisbee a lot right", what ever. Now she'll probably just laugh off what you said and sit back down with her friends. This is the best bit, just do the whole thing over again. Throw the frisbee at them and say "sorry, can you throw my frisbee back again please, really sorry". At this point you can either strike up conversation or ask if they want to play frisbee. From there in it's down to you. But trust me it works. The reason I say use a frisbee is because a.) girls arent too good with throwing or kicking balls, b.) a frisbee is more sociable, not many girls will agree to a game of ball but frisbee they would c.) when you first ask them to throw it back they might not giggle to begin with because you might have been playing with it while you were talking to your wing and it got away from you, with a frisbee its just obvious you werent playing with it so makes it funny.
If all you got is a ball then use the ball to get their attention. Either way its a good way to get them to acknowledge you and you then have an excuse to start talking.
Try it.