Don't you hate it when...

You've put in a million hours at work trying to help somebody out...then they ask you, very casually, what you're doing on Saturday? Only to try and trap you into working for them on the weekend. Forget it man! You can't have any of my precious Saturday time. Quick...got to think of a plausible excuse. I should always have one ready to throw out there when I need it. Blast! Need to be quicker on my feet. Um...I'm "washing my hair", my "dog ate my weekend". Got any good ideas?