Don't -ever- expect anyone to contribute for free. When they do, treat them special instead of taking them for granted.
That seems like common sense, but that's what I see all the time when it comes to programming projects, open source or not.
Right, but apply the same measure to developers too.
We often get into the habit of taking some contributor's work for granted, be we shouldn't. Every patch should be treated with the same respect and gratitude. If it's somebody's seven-hundredth patch, you don't have to say "thank you" for every patch, but you should still keep that humble attitude.
WINE's release emails always specified who did what, and that's not quite a 'thank you' but it's definitely more acknowledgement than I sometimes see.
It doesn't even have to be the project leader or other developers doing the praising. A proper bug reporting system will give the users a chance to give that praise as well, thought the leader/developers should also be thankful for the contributions.
It's not easy, and I'm not trying to say it is, but it needs more attention.
Just in this context of getting people to 'contribute', we're explicitly referring to projects where the developers do not get paid.
If you're developing software for somebody else, they're not paying you, and you're not getting any benefit whatsoever, that's entirely your fault. It's certainly not unfair though.