That said, doing what Xavier recommended is great advice as a cold approach (and good examples of not giving a fark). Also visiting the store with a female friend is helpful. A lot of times when walking into a clothes shop with a girlfriend the sales assistants give out huge IOI's. No matter what, you still have to get around to impressing the girl and getting the number/contact. There is no secret line or length of time needed - it's still down to your naked confidence, potential virtues and raw physical attractiveness.
Here's the thing about cold appraoches though - I don't really ever do them unless they are genuine situations where I randomly meet a girl or I'm introduced to a girl through work or a friend. If you plan a cold approach you are essentially still engineering to get towards a conversation that leads to a number and hook up. Why bother? It's hiding away. It's much braver to just go up and say 'Hi I'm____, I don't want to bother you at work but I'd like to get your number' (You are demonstrating being courteous and forthright). If she says 'why?' say 'Because you look like an interesting person' (she has to qualify/prove that she really is interesting plus she already knows why all guys approach and therefore you must have done this and succeeded before - prequalification). If she tests you again 'how do you figure that?' you smile and say 'I can see it in the sparkle of your eyes' staring directly into her eyes with 100% conviction (you're subtley communicating she is beautiful, you can potential see into/protect her soul but also that YOU are the one with the intuition and intellect to recognise it in such a poetic way - YOU are actually the desirable one). If she is single and does not find you physically repulsive, she will have to have an emotional (possibly even heart melting) response to this. Who does that? No-one. You are unique and special. If she says 'no', she is either not single or you're probably never going to be her type anyway. It's just a number. You just smile say 'ok' and walk away into the sunset to the next target. Her loss. If she says 'yes, here's my number' you can build further attraction later via texting. Eitherway she will have to concede a number of things: you are confident, you are brave, you were polite and friendly whilst being cocky enough to directly ask the number, you were honest, you are unique, you are mysterious. They are all attractive virtues demonstrated without much verbose, time wasting or interference with work.