Don't do this...
"We should definitely hang out again soon."
"I'll call you." <--- Granted these days it seems like everyone knows that's just a line, but still...
"I had a great time!"
"I like you but my friends didn't really like you." <--- Yes, I actually got that one once upon a time.
Do this...
"Hey, thanks for meeting up with me, but I don't really think we'd work as friends. Sorry about that."
"I'm flattered that you're into me, but I don't feel the same way. I'm very sorry about that."
"We can meet up again if you want to...but I really don't think this will work out. Here's my number, but please don't get your hopes up."
There's a lot to be said for being apologetic (or at least acting the part) and showing that while you may not be interested in pursuing anything with the person, the fact that you're probably hurting them with your words at least matters to you a little bit.