I'd say (but I do not maintain large FOSS): - Ask for donations - Ask commercial users to pay a fee (still voluntarily though) - Offer yourself as a consulta

I'd say (but I do not maintain large FOSS): - Ask for donations - Ask commercial users to pay a fee (still voluntarily though) - Offer yourself as a consultant on the software, you can help businesses with implementation or migration. Of course, there is a thin line. Having people to pay for your software raises expectations. Also, paying customers could steer you into a direction you (or the FOSS community) don't like. In my experience, almost all businesses are perfectly willing to pay for the software that make their business possible. Businesses understand that FOSS is free speech, not free beer.