"Thanks, but I'm seeing someone..."
or, if we've gone out once and I don't have any interest when the second date comes up;...
"I had a good time meeting you, but I've become exclusive with someone else since we last went out"
or
"Thanks, I really enjoyed meeting you, but I just don't feel we have enough in common/enough chemistry to take this any furhter"
I HAVE learned that honesty--re: that last response is the best, because they feel like, well at least she's being honest AND it keeps them from calling again in a few weeks to see if I've stopped seeing that other guy.
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