Don't do anything for free.
It could be seen as ultra-cynical but the saying from Yorkshire is true "If thee does ought for nought , make sure thee does it for thee sen."
In layman's lingo, "If you do something for nothing make sure you do it for yourself."
Never a truer word was spoken.
I wrote for several anthologies over the last decade, and while other writers were offered contracts and had book sales I got nothing; the only thing the anthology work did was ruin my Amazon ratings.