Sideboard notes:

You can basically break down your sideboard into the following cards: extra removal, extra countermagic, and random hate cards.

Extra removal: Magma Spray, Electrickery. Generically good when you want to play to the board.

Spray is very good against most creature decks as an additional shock. It gets the nod over other variants for its exile clause, which is crucial against Kitchen Finks and Voice of Resurgence. It gets the nod over Pillar for Instant speed, despite not going to the face. You should also be boarding this in against Burn as an additional way to fight their best cards: their creatures.

Electrickery is your ace in the mirror, since it is the only card in your colors that will deal with a Pyromancer and his horde of tokens while preserving your own threats.

Extra counters: Dispel, Spell Pierce, Negate. Good against combo and control opponents.

You board in these cards as you see fit against combo and control decks. Spell Pierce is much better on the draw than Negate, since you can setup on turn 1 with a Serum Visions into turn 2 Delver + Pierce, flip on turn 3 and bash. Dispel fills a similar role, since much of Modern's removal is instant speed. It is also the only counterspell I would board in against Burn, since it is not an inherent tempo-loss against half of their deck (1 mana vs 2 mana). Negate is just a catch-all and is better against control decks since it doubles as a counterspell for their threats (namely Keranos, which you can never beat if they untap with it at a reasonable life total).

Random hate cards: Relic of Progenitus, Shattering Spree/Vandalblast, Dragon's Claw, Blood Moon.

It should be obvious when you board these cards in. You should probably board in Blood Moon less often than you think, though. It's good at locking out Tron and greedy Scapeshift players. Don't bring it in against control unless you are on the play in game 3 and they have demonstrated greedy mana-fetching in both games prior.