You should probably main deck Tempered Steel, instead of side board.
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Check out my Myr deck (http://www.mtgvault.com/ViewDeck.aspx?DeckID=171013), you may want to add Tempered Steel, all of those little Myr are easily targeted by Black Sun Zenith, Tragic slip, Virulent Wound. You also need a way to avoid attacking creatures that fly, those are BAD for Myr decks.