One of the best things you can do is no matter what happens when you walk away from an interaction, ask yourself what YOU could have done better. It doesn't matter what reason the girl gave for not wanting to give you her number/go on a date etc. If you buy into the girl's excuses you will probably miss a lot of lessons and opportunities for improvement.

A good thing I like to do to see if I'm kidding myself is use the Brad Pitt rule. In other words imagine if the girl's excuses would make sense if you were Brad Pitt.

"I'm sorry, I just don't give out my number to anyone..."

"I'd love to see you but I'm goldfish-sitting on Friday night..."

You get the idea.

http://www.artofmanliness.com/2008/02/05/the-brad-pitt-rule/