I just automatically refused too many times to go out with friends. I used to say: "it's not my style..." (Witch is true, I don't drink and I don't like those things).
One time I accepted the invite to go to a bar (the only one I've been) (it was underground, very dark and crowded), he was just grabbing something from a friend (if I remember right) and we stayed a few minutes waiting for the guy. We seated at a table and he gave me a beer. I said no (I don't drink) and he had to drink both. The guy arrived and the time "was just passing too slow"... all I wanted was to get the hell out of that "bunker". I was just looking at the walls while he was talking and drinking (slowly).
It was very close to the school, so I heard the bell ringing and I said to myself: "thank god, I'm out of here." :x
One other time one old friend of mine asked me if I wanted to go watch a movie at the cinema. He insisted very much and I was very nervous to say no. I accepted. :? And I knew he was going with some friends! 8O
I phone him later to cancel, saying I couldn't go, but he already had the tickets.
I couldn't go... really, I just didn't. I said to him to invite some other friend of him but I think he didn't found any in such short notice.
So, today my "friends" already know that I refused to go out, so they don't even mention it to me. Which is not so bad in one perspective but I also lost contact with some of them over the years because of that.
It's a mixed feeling, because I really don't like bars, discos and places like that. The downside is really the fact that I don't have "common interests" with them and with time I just loose contact.