Most Helpful Girl
I don't say that anymore because I've realized how much it upsets guys, but when I did, most of the times I didn't really mean let's actually be friends and hang out together. It was more like... "I value you as a person but I'm not romantically interested in you so let's move on with no hard feelings" and my way of trying to convey that and cushioning the blow would be to say "let's just be friends" (who soon drift apart and don't talk to each other anymore).
Like I said though, I avoid the whole "friends" spiel now.
Most Helpful Guy
I had it done to me recently. I politely told her it been nice meeting her and hanging out with her (we'd gone out three times) but that we were looking for different things, so good luck.
I was pissed off about having had my time wasted but at the same time, we'd had some fun so I wasn't going to be a jerk about it.