I used to have big problems with this but It will get easier as you get more confidence and the more you don't care what people think.
Then you just blurt out whatever comes into your head, if you think "what will she think if I say this" then it's probably going to come across as forced.

That's why I think canned stuff works, not because of the content (although there is some merit to it) but because it comes with a seal of approval from someone who is good and has a track record of being successful. So therefore you don't feel as conscious or "think will this work?" because you know it works because a pickup artist is recommending it.

You can just bypass this though by having a strong sense of self and not seeking approval from other people. Also I've noticed when I just ramble random shit if the girl's my kind of girl she'll join in and be whisked away to my ace random world of stuff, if she's boring and dull it'll be like pulling teeth but even then you can still get somewhere.