* Don't work on FOSS that supports a SaaS or proprietary product. You're enriching someone else's ecosystem for no money. You take on the communication bandwidth and issue load of dealing with not only your issues but their issues too. Your software is useless if theirs fails. It's thankless.
* Don't work on FOSS you aren't using yourself anymore. This heuristic isn't fully fleshed yet, but I've found if I stop using software I wrote, it just becomes a support burden for me. Especially frustrating are entitled users or entitled users who make money entirely using my software who want niche issues fixed. Not PRs, they want me to debug it all or put in the thought to get the features working. (Probably doesn't apply if you're immersed socially with a friend group working on a FOSS project though)