Here are some ideas when looking for key cards to stop those decks that you cant seem to beat.
Chain Disappearance - This is the best sideboard card EVER!!! Stop Dragunitys, Six Samurais, Dandy Quickdraw, Chaos Plants, X Sabers, Frog Monarch, Fish OTK.
Light Imprisoning Mirror - Stop Lightsworn and Chaos Decks.
Dark Imprisoning Mirror - HINDERS blackwings, Chaos, Stops Gravekeepers.
Rivalry of the Warlords - Greatly consider this card in certain Decks! Stops Dragunitys, Gladiator Beats, Chaos, Dandy Quickdraw X Sabers, Frog Monarch, Fish.
Mask of Restrict - An old LON Card but not forgotten as was remade in Gold Series. Stops Frog Monarch, Black Wing, Plants, Fish.
Starlight Road - Is never expected! Stops decks that like to go Boom! Or likes to Synchro for Blackrose. best used against Gladiator Beats and Blackwings.
System Down - Always useful against the machina match. As a downside you have to pay 1000 Life Points
Cyber Dragon - Useful against the Machina decks, But can easily be negated and does not removed machines from the graveyard.
Gottoms Emergency Call - Really just a great card against the X Saber Match up!
Forced Back - This is my Idea! a lot of decks are based off that initial normal summon and Forced Back may seem a little bad but its like a compulsory! Decks that are hurt by Forced Back, Xsabers, Blackwings, Gladiator Beats, Dragunitys, Machina, Plants, Fish, Gravekeepers.
Seven Tools of the Bandit - A lot of sideboard cards are traps. This card helps you keep the wining edge when you side this in against their sideboard trap cards.
Swallow Flip - It stops most of the Gladator beasts by negating there effects and destroying them, Stops the dragunity synchro, Formula Synchron, Hyper Liberian, Black Rose Dragon, and Even Groz
Drastic Drop Off - Hella good! Punish their search cards. A lot of decks are dependent on searching their deck for that one card they need. Stops Six Samurais, Machina, Gravekeeper, Dragunitys, Pot of Duality, Gemni City/Hero Deck.
Mind Crush / Trap Dustshoot - They are just amazing and work the same as Drastic Drop off.