I have a couple of thoughts on this... First off, the "working for free" things should really only be leveraged on your first or an early film where there just is no other way to make it work. I've always been of the opinion that you should pay at least a little bit, even if it is a token amount, and it changes the dynamic slightly. As my career progressed, I never asked for things for free, it didn't seem professional. That doesn't mean I didn't beg for discounts but people have skills and abilities that should be compensated for.
The other side of it is this... I don't work for free anymore. Especially if this is a real company or they pull the old "work for credit" thing, pay me or I'm unavailable. If I like the project or I am supporting someone I will volunteer to help and realized I probably won't get a return favor. I hope for the best, expect the worse. I also learned to not be bashful about asking for a return favor and really hold it over them. 'Hey buddy, I did you that solid, I need you for half a day next week!'