I approach sideboard this way:pick a match up, say shops go thru my deck and find the cards I dont want in the shops match up: example -2 flusterstorm, -1 mana drain, -2thought cast. That gives me 5 slots choose 5 cards I would want most against shops repeats steps 1-3 for any other match up I'm likely to see in a given meta evaluate my card choices for sideboard and find cards with duplicate match up uses. Example I may bring in fragmentize against shops, oath and jeskai control (I often find jeskai control running null rods and/or stony's). I would use containment priest again oath, dredge and tinker decks. So I go back thru with this kind of evaluation and add and subtract cards until I'm at 15.