This is what happened couple days ago. In a store I met 2 hot girls from my school. We left the store at the same time and I was dying to come up with something interesting to say. But the only thing I was able to talk about was school and very common but boring topics.

And afterwards I was angry to myself not being able to build an interesting conversation.

I often find myself in similar situations. The material (tricks, games and other stuff) is sometimes useful, but there are a lot of situations were they won't fit and one should come up with something funny/interesting/crazy/flirty related to the situation or very general that everybody can relate with.

Any advice?

Personally, I think if you go into a situation with canned material, you're destined to fail. It's inevitable that you will run out of material, then revert back to old, AFC average conversation.

The only way to improve this is to go out and talk to girls with no specific goal in mind. Just overcome your anxiety and be friendly. If you're comfortable in a social situation, your results will speak for themselves, and THEN you can go in with the mindset of picking up a girl.

Wit is a difficult thing to teach, but the more you talk to girls, the more you will find out what makes them laugh, what pushes their buttons, etc. Practice practice practice and try not to be predictable. Don't try to impress them; talk to them with the mindset that you're doing them a favour by gracing them with your presence (try not to cross that fine line between cocky and arrogant).

This is usually my problem as well. Having a fun spontaneous conversation can sometimes be hard. Sometimes for me though it just clicks, and it's easy. But a lot of the time i just go back to the AFC conversation. I have yet to find the reason why sometimes it clicks, and sometimes it doesn't though..