Confident Men Don't Explain
"I can't come because..."
"I can't do that because..."
Most people, when refusing something, will immediately add, "because..." and launch into an explanation. But they didn't ask for an explanation!
Even if they'll want to know, you can come off so much more confident by putting forward the refusal and then saying nothing - forcing them to ask why.
The message it gives is that you could really give a damn about what she thinks about you - and whatever you do you know will be right, so you don't need to explain yourself.
The trick is also an "open loop" as described in Swinggcat's book, Real World Seduction. It involves them in the conversation by making them ask questions to open you up, and in doing so commits them psychologically to being interested.