Building Your Side Deck

Building a side deck is necessary if you want to make your deck as competitive as possible. A duelists ability to adapt mid-match against their opponent can make or break them.

Step 1: know your deck
This is more than simply saying: "I run X-Sabers/Blackwings/Six Sams/Flamvells." You also have to be able to say: "okay, since I run Flamvells, I get an okay matchup against decks that swarm since it gives me Firedog targets. I am slaughtered by decks that run Skill Drain and Macro." Analyze your deck's strangths and weaknesses, and begin your side deck with things that counter those weaknesses you see.

Step 2: know your meta
This step is a bit more difficult, since the meta is constantly evolving. However, taking a look at which decks have recently topped at YCS tournaments will be able to give you some idea of what will be a popular match-up, and therefore what you can side against them. For example, a Blackwing player can easily side Gozen Match and Rivalry of the Warlords against lots of decks.

Step 3: select possible side cards
Put cards that will help solve the weaknesses of your deck, exploit the weaknesses of others, and add to the strengths of your deck into a pile to begin your side deck

Step 4: narrow it down
It's very likely you have more than 15 cards at this point. Divide the stack as equally as you can into five piles, then of the five piles select 3 from each that will be most beneficial to you.

Congratulations! You now have a side deck!

Bonus A: Transformation Siding
There are many decks that have a similar core skeleton with a difference of 15 cards or less. In cases like this, siding can take on a secondary effect of throwing your opponent completely off-guard, since you can take advantage of the fact that they will be siding their own deck to counter your original strategy.
Examples of such siding:
•Watts can easily side from a Messenger of Peace lock-and-poke to a full-on Solidarity Stun beatdown.
•By teching Elemental Heroes into Fortune Ladies, Absolute 0 becomes easy to summon, and you can side the Fortune Lady engine out to go into E-Hero beatdown
•There are many Exodia styles, and by choosing two of them, you can side from one strategy to the other quite easily throwing off your opponent's ability to counter you.

Bonus B: Shadow-siding.
If you use sleeves, make sure you have enough of the same kind for both your main and side decks. Between duels, shuffle your side into your main, then go through and pull out the same number of cards you had in your side in the first place. Your opponent will now have no clue as to A) how many cards you sided and B) what they were. Happy dueling!

I love shadow siding, it is the only thing i do when side decking.

and transformation sideboards are awesome as well

Shadow Siding is a tool in my arsenal as well. I find it to work very good.

Had a guy at my shop tell me about it. I really like the idea, so I included it here. Most people on my team(s) that read this hadn't heard of it before, and they'll be bringing the idea to their respective locals.

Well, it's effective, legal, and usually rather simple.

You should do it rather fast though, as to avoid stalling or anything of the like.