1) Don't say yes to a guy when you don't want to do something, or aren't interested.
2) Don't say no in such a way that the guy still thinks he has a chance to ask you the same thing later (like going out on a date).
3) If you have to cancel a date/event for a real reason (you really are sick) but really are interested in the guy, don't leave it up to the guy to re-ask you out again later; we tend to think you were just trying to be nice about standing us up. Now that you know he was interested enough to ask you, it's okay to ask him out when you are feeling better, or whatever the case may be.
4) Don't tell a guy you will call him, and then not call him.
5) If you are cancelling plans because you didn't follow #1, don't do it 1 minute before you were supposed to head out. We prefer a few hours notice so that we can make other plans.
6) Standing up a guy isn't good. If you didn't want to do something with that guy, follow #1. If you didn't follow #1, then follow #5. Call/tell him with plenty of notice.
Sometimes I hate it when people say they want some photography done, then don't even bother to show up or even call to say they can't do it.