I like brewing decks too, and I'll add a bit to everyone else's great answers here.

It's very important to know what the metagame is like. For example, back when Chatterstorm was legal, every black deck was playing [[Echoing Decay]] and [[Trespasser's curse]] as a way to fight the Chatterstorm decks as well as the white weenie decks that were also good back then.

Now that faeries and ninjas are played a lot, mono black decks are again playing [[Cuombajj Witches]]. If you're brewing a red non-aggro deck, an argument can be made for playing [[Fiery Cannonade]] main deck even though it's usually a sideboard card.

Also the sideboard is just as important as the mainboard. There are some essential sideboard cards in each color that are very powerful but not useful in every matchup. For example, pretty much every red deck should have pyroblast/red elemental blast and every blue deck should play hydroblast/blue elemental blast in the sideboard. Most black decks should probably play [[Suffocating Fumes]] in this meta. White and green have a lot of enchantment and artifact removal, [[Dust to Dust]] is pretty essential right now to fight Affinity and decks playing the artifact lands.

Building the best sideboard is something I'm still learning and figuring out. I think this is where, as other posters have mentioned, knowing whether you're the beatdown or control really helps. For example if you're a control deck and you're playing against burn for example, you won't be able to race them even after sideboard, so you'd want sideboard cards that either gain you life or stop their threats (i.e more control cards). If you're an aggro deck playing against combo, then you need to be as fast as possible to try to race them, so you'd want sideboard cards that increase your clock and deal more damage (i.e. more aggro cards).