One thing people don’t always think about is that even though may not want to break down your carefully crafted deck, you do actually play more games with sideboard available than not in tournaments. This seems to be a problem especially among newer players, who don’t often have any sideboard whatsoever.
Simple math to show this: If you lose all your games, you still play one sideboarded game for each first game of a match. Since you do (hopefully) win games here and there, you’ll generally play at least a bit more sideboarded games then not. Since many matchups depend highly on who’s on play and who’s on draw, you’ll probably play about 1.5 sideboarded games for each game with only mainboards.
Therefore, a good sideboard is important.
Not that I know how to build one… and as I’ve said before, that won’t stop me from talking about how to make one.
First, back in the day we had plenty of color hosers. We had [scryfall]Gloom[/scryfall] for white and [scryfall]Karma[/scryfall] for black, among bunch of other such cards. In Alpha each color had a good hoser for each of its enemy colors. These were often extremely powerful, yet narrow. In those days making an effective sideboard was easy. However, Wizards stopped that. Getting hosed in such a way wasn’t fun, so even though they have delved into that territory since then (such as the cycle of rare creatures in M14), these cards have been scaled back, which makes the game much better.
However, it also makes designing your sideboard more of a challenge.
Take Out All the Cards You Don’t Need
I feel this is actually more important than finding good cards in your sideboard. Exchanging dead cards for something that is even marginally useful is very good. Back in the last standard I ran [scryfall]Doom Blade[/scryfall]s in my deck, as did my opponent. I could exchange them to something else, he couldn’t. Points for me. Also, back in the last standard, I would take out all the mana elves from my deck and put more midrange creatures in instead against control, because the elves were just collateral damage when my opponent [scryfall]Supreme Verdict[/scryfall]ed, whereas I had some resilient creatures in the sideboard.
Different Cards for Being on Play and Being on the Draw
This is especially important in mirrors, although not many people do it. In a mirror matchup, you need to think about how playing and drawing will affect your position in the game. Usually the player on play is obviously going to have to use the momentum to keep the player on draw on his toes, while the player on draw wants to be able to answer whatever the opponent throws at him. Therefore, in these positions you might want to move your deck into a certain direction.
Fighting Specific Cards
Sometimes you just can’t answer certain cards and you will just lose to them. At the moment, as I play a lot of black and white, my nemesis is [scryfall]Blood Baron of Vizkopa[/scryfall], although I have never actually seen it on the battlefield against me. I do know people at my LGS do play it and I’ve played against those players, but I guess I’ve just been lucky. However, I need to be aware of the card, because it will just kill my black or white deck. Finding good answers is easy enough. White has [scryfall]Celestial Flare[/scryfall] and black has [scryfall]Devour Flesh[/scryfall]. Both are also fair against other cards, so they are autoincludes in the sideboard.
Fighting Specific Strategies
At every LGS there are those few players who will always drag the FNMs long by playing Esper or U/W control. Those are good archetypes, which are viable no matter how the metagame evolves, because at this level many players don’t know how to play against control. On the other hand, you can’t design your deck solely against those control decks. So, you need to put something against them in the sideboard. I like cards like [scryfall]Ranger’s Guile[/scryfall] which always gets those players by surprise. Anything aggressive is good, anything resilient is good.
Nextleveling Your Opponent
Always think about what your opponent might board in. And if they are any good, they will be thinking about what you will be bringing in. Lets say your opponent is playing blue with [scryfall]Nightveil Specter[/scryfall] in the deck and you are playing black. Are you bringing in [scryfall]Dark Betrayal[/scryfall]? What if your opponent next levels you and takes out all the black cards in his deck?
Which brings us to transformational sideboards. Recently Stanislav Cifka was playing a U/W Control deck with a twist. There weren’t any creatures in the main deck, besides the ones [scryfall]Elspeth, Sun’s Champion[/scryfall] and [scryfall]Mutavaul[/scryfall] can provide. So, generally the opponent would take out all creature removal… while Cifka would actually bring up to eight creatures from the sideboard.
This idea isn’t really new and it has been done before. It can be a devastating tactic. Also, it can get you more wins as a control player if you don’t do it, as the opponent might try to nextlevel you. Just remember to at least fake a lot of changes between games.
Your deck relies on it doing certain things. If you don’t let it do that, it won’t win. Therefore, even if you see potential in many of the cards in your sideboard, if you can’t take out more than, say, four cards, don’t force it. Of course this highly depends on multitude of factors, but don’t make your own deck weaker by taking out too many cards.
Also, on the other hand, the sideboard is a very limited resource at 15 cards, so you shouldn’t put too many cards that specifically against a single strategy. In Legacy, you have to take the Dredge decks into account somehow, but you can’t just stack your sideboard with gravehate.
Use What Fits Your Strategy
Say you are playing [scryfall]Varolz, the Scar-Striped[/scryfall] in your deck and you feel you need something to destroy artifacts. Well, since you can abuse your creatures in the graveyard with Varolz, maybe you should look into using using something that is a creature for this spot. Maybe [scryfall]Keening Apparition[/scryfall]? Same goes for many other cards. Apparition is much better with your [scryfall]Heliod, God of the Sun[/scryfall] than [scryfall]Ray of Dissolution[/scryfall], because it enables the Heliod.
Sometimes Its Just Better to Not Sideboard
Sometimes haters are gonna hate and you can’t really do anything about it. Then you just need to sit back and see what happens. If they have it, they have it. If they don’t draw that uberhate against you, than go ahead and pummel them. Don’t wreck your own deck just because you feel the need to board in something.
Try to Find Cards that Are Good Against Multiple Strategies
Lets say you are playing an Orzhov deck and you feel you need to put in something for the [scryfall]Nightveil Specter[/scryfall]. Yes, we’re thinking about that card again. Actually, in my experience, people just maindeck stuff for it, because its pretty ubiquitous. Anyway, you might think you want to put [scryfall]Dark Betrayal[/scryfall] into your sideboard for it, but how about [scryfall]Last Breath[/scryfall] instead? It does answer the Specter, even if it does give your opponent life, but it also answer a lot of other creatures, like [scryfall]Voice of Resurgence[/scryfall] which has seen its own resurgence recently. Also, in that same G/W deck: [scryfall]Experiment One[/scryfall], for example and probably other cheap, low powered creatures.
Looking at the Whole 75 as a Deck
Sometimes your deck doesn’t have many synergies and is more based on good stuff. Then, you might just want to shuffle your sideboard in and take out fifteen of your worst cards in the matchup. This might help with cards like [scryfall]Lifebane Zombie[/scryfall] because you might not think to put it in your deck otherwise, but it might be good against players other than those playing green and white, because intimidate is still strong and looking at your opponents hand is often good value.
Lands in the Sideboard
If you are going transformational or just want to change your emphasis after boarding, you might think about putting a couple of lands into your sideboard, or if you are taking out costly cards, going the other way and taking a few lands out. Some people do this based on whether they are on the play or the draw. You sometimes need less land on the draw (especially in limited with 40 card decks).
Sideboarding in Limited
Well, you don’t have much choice in limited, but card values change based on your opponent. I don’t like to play [scryfall]Viper’s Kiss[/scryfall], but I will bring it in if the opponent is playing [scryfall]Scholar of Athreos[/scryfall] or maybe [scryfall]Erebos’s Emissary[/scryfall]. Although those values might not change much, you still have plenty of room to maneuver, if you’ve drafted enough good cards for this purpose.
Those values might be harder to identify, especially since you haven’t had much time with your deck, so you don’t know exactly how it behaves. Therefore, drafting a lot would probably help. You need the instinct to be able to use the limited time you have to find the right configuration in your deck. On the other hand, often you have so little choice, it takes no time at all.
The Skill I’m Missing
As I said in the beginnning, I’m not really a good sideboarder. I tend to not utilize it as much as I should, but also, I’m not good at building a sideboard. The problem is that I don’t know the current metagame, and if I did, I wouldn’t know how to answer it. My card knowledge is somewhat limited. I know Theros very well, as I’ve been drafting it quite a bit, so I have good grasp of what’s good against what, but I just don’t have the same grasp when we’re talking about the whole of standard.
Obviously, this is all pretty shallow and to really be good at sideboarding, you need test those… which I never do.