Since WIA posted something more elaborate I have to compete
OP, if you seek some good universal rule or kind of a guide then here's mine to you.
You can take initiative 3-4 times. So whether you want to approach or move her a round or take her clothes off you have around 3-4 attempts to do it. After that you go into desperate/needy/pushy mode. Remember: Those attempts have to come across normal casual and rather polite. If you cross the line by saying something very stupid early on she will close up and no more attempts will help you.
So let's take an approach in daytime as an example...
You say something to the girl and answers "huh?" all confused. That is your
1st attempt. Thing is, first sentence is often not enough to break the ice with a stranger especially when they don't expect you to talk to them.
You know that and it doesn't bother you so you try one more time "oh hello am I interrupting something important? If I do just let me know, I didn't know".
2nd attempt goes better cause she's already snapped out of her by your 1st attempt so she actually hears what you say. And most importantly she sees you're normal guy who with some empathy. So she goes "um no it's fine..". Boom! Now she can finally listen to what you want to tell her.
Your
3rd attempt is "why it took you so loong?" (thanks again WIA) or "how's your day going? let's make valentine's day today". Whatever you want. Now the real interaction/communication can start. She can still be skeptic about you.
So you try for the one last time,
4th time "it's ok, you can talk to me, don't worry, I don't have a swine flu" to loosen up the vibe. Let's say she opens up and talks to you. Cool, you try, you stand up for yourself, don't back down under pressure, you're good. If she's not open to it after your 4 normal attempts and wants to leave you let her go. This is that moment after which you go into desperate mode.
It's similar with any other action you take. You want to move her physically? Try once, take different angle and try again later, take different angle and try again later, etc, until 3-4 times. Same with texting or trying to kiss [try few times here and there], trying to take clothes off, etc.
In general girls enjoy that chase as long as they feel good in the meantime. Besides, almost all of them are very super convinced that even if they want the guy they can't just say OK right away cause that makes them easy.