you can say you are exhausted and don`t feel like going out tonight and stick to your guns. nothing wrong with that. you have other plans, this is grounds for choosing to stay behind, oh well. it happens, with people having their own lives and all. eventually as you are more consistent in saying no and sticking, they will get the into the habit of you not changing your mind when you opt to not go out. if you are feeling like things are getting old with your friends, maybe you can pick something that you would like to do and they would not mind doing as well. overall there is nothing wrong with deciding to stay in and nothing wrong with asserting it either.