my advice is this. Picking up girls the way we do is just like being a business man. I'm going through a training program now where they make you eat rapport for breakfast lunch and dinner. Having said that, you need to remember that sarging women is not unlike selling and one of the number one rules of selling is that its easier if you make it more personal. If you are in front of her at a bar, thats when it is hardest for her to say 'no' believe it or not. If you're on the phone, it becomes a little easier, and if you're texting/emailing/IMing her its very easy for her to sit down and think of an appropriate way to say 'fuck off.' So of course the best option is to force her to see you again because once your there and making her laugh, she'll forget why she ever wanted to cancel.
Also remember that it will be harder to get a second date with drunk girls because their inhibitions come back during the hangover. I would try to avoid girls that are falling off of barstools or spilling drinks.
But definatly avoid texting. If you're thinking she'll say "why the hell is he calling me" remember that SHE gave you HER number. You didn't steal it (technically

) Remember, you're the fucking man, and the fucking man isn't afraid to call some bitch and ask her on a date.