More often than not by working a full-time job somewhere else.<p>Besides that, there are support contracts, sponsorships and stuff like that, but I'd say there are not a lot of people able to live from that alone.
Donations, documentation books/ebooks, small jobs modifying code as a consultant.<p>Open Source and Free Software have an NPR/PBS business model sometimes corporations like Microsoft and IBM donate to them, sometimes they have to take donations from users.
Lots of resources for that here:<p><a href="https://github.com/fossjobs/fossjobs/wiki/Resources">https://github.com/fossjobs/fossjobs/wiki/Resources</a>