Street PU (MT) are probably the hardest to realize - because they are MOVING TARGETS. They are unpredictable and probably the most ballsy approaches you can make (a bonus for confidence). A street PU pushes you to STOP a passing woman, break her state, create the state YOU WANT and attempt a PU. If this was not bad enough, there are many factors that will influence whether you are successful or not.

1) Weather - is it raining? Cold? Snowy? Are you causing the girl to stand in the freezing while the PU is attempted?
2) Time Constraints - is she in a hurry? Is she on a lunch break? Catching the bus or train?
3) Friends - Is she with friends? A group?

I believe the best way to break the state of a Moving Target (MT), is to compliment her. You need something to make it worth her while to STOP. Will she stop if you try some fluff talk? I think complementing a girl on something - energy, clothes, special piece of her ensemble etc is the best way to break the state and hopefully get her to continue the conversation. Then the next best thing is to PACE HER REALITY. Tell her that this is a strange way to meet someone - on the street, you know that she is going somewhere, in a hurry etc. Now RJ says it is good to say 'STOP' to put her mind on hold and clear a blank state - this works if you are up to it!
Also, try to approach the MT on a 45 degree angle - so that they can see you out of the corner of their perspective. Approaching straight on is CONFRONTATIONAL and from the back is MUGGING.