More often than not by working a full-time job somewhere else.
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.
orionblastar
Donations, documentation books/ebooks, small jobs modifying code as a consultant.
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.
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.