1. It matters a lot if you are on play or draw. When you are on the draw, you are behind in the tempo and damage race from the start. However, you do have one extra card instead. As a result, you can safely cut a land and shave a few cheap threats. Meanwhile, you should add efficient answers and catch-up cards. Fatal Push and Radiant Flames are examples of spells that are better when you’re on the draw, as they allow you to steal back tempo.
2. Sideboard against post-sideboard deck. Bringing in cards that would have been good in game 1 is a common mistake. Try to predict a gameplane of your opponent in game 2 and adjust.
3. Don't oversideboard situational answers. Sideboarding a card, that blocks only up to 4 opponnent's cards it is not a good decision. Your card will too often be a dead card. It should have a least 8 targets.
4. Don't oversideboard in general. Sometimes, by bringing in too many sideboard cards, especially reactive ones like removal spells or counterspells, we run the risk of simply diluting our deck’s core strategy so much that we hurt our chance of winning the gam
4. If you are not familiar with deck your are playing, becasue you copied someone's else deck list, you should prepare your sideboard plan before tournament. It is legal to have notes what you should cut and what add in specific matchup.