what are some of the absolute worst things you could say or do to a girl? things that will turn her off and ruin your chances?
I will start:
qualifying yourself. for example saying things like "I am usually dressed much better, sorry I am not dressed well today"
staring at her like a moron.
now you go. I think this thread will be pretty entertaining!

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I have not failed 10,000 times. I've just found 10,000 ways that won't work. - Edison

being yourself, as PUA would have it

Ima say letting there be an awkward pause moment and not filling that gap with some conversation. Females think that it's our job to entertain them like we are some kind of court jesters...and that's fine. Never mind that they could equally fill the awkward pause...as we all know they sure can jaw it up with their friends.

This awkward pause, especially a long one, can and will make them uncomfortable. And the thing that is at the top of their list before coming off some nookie is that they need to feel comforted/comfortable....by a strong (positive) authoritative male figure.

Another good communication tactic is don't let her talk about her ex'es...at least not very long and then change the topic quickly. A few tid-bits of her speaking of her ex can give you some insight into her personality and her map and model of her world that is useful...but don't let her do "My ex BF this, my ex BF that..." shit on you...or ignore you by texting. Both of these are shit tests. Her expecting you to be the sparkling conversationalist is also a shit test to a certain degree as well...so pass it and don't let there be an awkward pause. Worst case scenario...if there is an awkward pause, ask her what it is that she is passionate about and get her talking about herself a little bit. Keep the conversation flowing.