In this world the trouble of saying "yes" all the time, as mentioned above, is that your employer thinks you are a soft touch. And then the one time you say no you end up with a black mark against your name because they just expect you to do it.
I haven't got the flying job yet but in my present job because I live closest to wear I work, whenever an extra bod is needed in a hurry to help out I get the call first. Yes I like the extra bit of cash that comes now and then but as a result sometimes you do get taken for granted. It pays everynow and then to say "no". You can come up with any sort of excuse ie - "it's my day off and I have been down the pub this afternoon" is one of my favourites.
Crewing have got to be like the taxman really - everybody hates them but they are only doing their job.