I usually sideboard the following things:
Graverob / Purify / Rancid
Single target / Command removal (kidnappers / kidnapped by demons etc.)
Then there is some niche stuff that differs by faction that I can carry in sideboard.
Examples include: ancient aether (WP), shattered veil (GI / Ex), disruption sphere.
Of course there are some exceptions here and there. At least 1 calamity should be in mainboard for 2/3p FD probably. Also single target/command removal you should probably have at least 1 of in every deck. And then there are certain cards like consecrate which are so multi-purpose you should probably not carry sideboard-only.
I am a large fan of limiting myself to 40 cards in the main deck to get the most consistency. I've heard people say they'd rather go over 40 cards to get some of the stuff I typically sideboard into the main deck, and then throwing the stuff out they don't need during the sideboard. I can imagine they will have some dead draws in the first match, which may cost them that first match. At the same time, when I am facing a deck that relies heavily on artifacts/locations/grave, I am likely to loose the very first match too. I'd rather just keep my tempo up and have less dead draws though. But that's just a personal preference thing I think.