Also, anxiety and unnecessary pressure is what screws you up. You're putting too much pressure on yourself to close the deal instead of enjoying the person that's in front of you. She'll start to feel it when you're becoming too anxious about getting the number, and she will automatically not want to give it to you.
Talking to a woman and getting her phone number is only a first step, and so it's not a big deal if you blow it. It's not about how many phone numbers you get, it's about getting to the next step with the right women. Not all the women you meet are going to be worth your time, even if you DO get the number. Keep yourself focused on getting her set up for the next step.
Not every woman is going to want to take that first step with you, and only some of the women who do take the first step and talk to you, are not going to want to take the second step of going on a date.
Remember, your goal should be to get to know a woman, and then take it to the next step. There's no pressure. She's not there to reject you. If she likes you, then she'll gladly make it easy for you to take it to the next step... whether it's a first or second date, or going home with you.
Women are not in the business of rejecting you. They want men just as much as men want women. They simply go about it in different ways. The way we have things set up in society, men are the ones who are going to have to be the aggressor, and that's just fine.
Let go of any perceived "need" to get her number. Just relax, treat her like a human being who's genuinely interested in you, and whom you're genuinely interested in also... and things will go very smoothly for you.