This is a topic that has been covered extensively, so especially love it when I find something fresh to consider.
Never work for free is a difficult one. If you are just starting out as a freelance writer, for example, you may need to write a sample. I completely agree a sample should be paid, but I have had a client our two that turned out great that did not pay for samples.
I didn’t write the sample I gave them for free however. It was already in my portfolio. If you are just starting and do not have a portfolio, take a spare minute and put at lease a few pieces together.
If the work is done just once, you do not have to worry about taking the unpaid time again, and then you will slowly begin to build your portfolio as you work more jobs.