View the staff credits again:
To do this, you must have already viewed the staff credits at least once. To view the credits again, input the old famous Konami code by pressing Up, Up, Down, Down, Left, Right, Left, Right, B, A at the selection menu.

All Cards:
Beat every duelist 20 times, then at the title screen press L, R, Select, A, B, Left, Right, Up, Down, Start

Unlock the Limitation option:
To unlock the limitation option, get 100% completion by collecting every single card in the game. Now you can choose to have restricted cards to be no longer restricted.

Unlock Boosters:
To unlock these boosters, you must have ten wins against the specified duelist.

Zombrya the Dark Booster - Beat Mai Valentine ten times
Dark Ruler Hades Booster - Beat Bandit Keith ten times
Guardian Sphinx Booster - Beat Pegasus ten times
Yamata Dragon Booster - Beat Joey Wheeler ten times
The Masked Beast Booster - Beat Maku Tsunami ten times
Toon Summoned Skull Booster - Beat Espa Roba ten times
Thousand-eyes Restrict Booster - Beat Bonz ten times
Jinzo Booster - Beat Weevil ten times
Blue/Green Millennium Booster - Beat Strings ten times
Blue Millennium Booster - Beat Rare Hunter ten times
Relinquished Booster - Beat Rex Raptor ten times
Red Millennium Booster - Beat Arkana ten times
Gold Millennium Booster - Beat Odion ten times
Purple Millennium Booster - Beat Umbra and Lumis ten times
Green Premium Booster - Beat Simon twenty times
Purple Premium Booster - Beat the Duel Computer twenty times
Orange Premium Booster - Beat Yami Marik twenty times
Pink Premium Booster - Beat Yami Bakura twenty times
Blue Premium Booster - Beat Yami Yugi twenty times

Unlock Duelists:
Follow these instructions to unlock more duelists.

Espa Roba - Beat all tier 1 duelists 3 times each. Open at least five Red Eyes Black Dragon booster packs.

Mako Tsunami - Beat all tier 1 duelists 3 times each. Open at least five Black Luster Soldier booster packs.

Bonz - Beat all tier 1 duelists 3 times each. Open at least five Barrel Dragon booster packs.

Simon - Beat all tier 5 duelists 15 times each. Duel either Tea, Tristan, or Truesdale three times in a row and on the fourth time, Simon will be unlocked.

Duel Computer - Beat all tier 5 duelists 15 times each. Duel Seto Kaiba three times in a row, and on the fourth time, the duel computer will be unlocked.

Yami Bakura - Beat all tier 5 duelists 15 times each. Duel Bakura three times in a row, and on the fourth time, Yami Bakura will appear and be unlocked.

Yami Marik - Beat all tier 5 duelists 15 times each. Duel Marik three times in a row, and on the fourth time, Yami Marik will appear and be unlocked.

Yami Yugi - Beat all tier 5 duelists 15 times each. Duel Yugi three times in a row, and on the fourth time, Yami Yugi will appear and be unlocked.

Ishizu Ishtar - Beat all five Ghouls ten times each. You must use a deck without any magic or trap cards to duel Ishizu.

Seto Kaiba - Beat Bakura, Mai, Joey, Ishizu, and every Ghoul ten times each.

Mai Valentine - Beat all tier 2 duelists 5 times each. Have a Harpie Lady card in your trunk, and no Harpie's Feather Duster card in your deck.

Joey Wheeler - Beat all tier 2 duelists 5 times each. Have a Jinzo card in your trunk and none in your deck.

Marik Ishtar - Beat each of the five Ghouls once.

Rex Raptor - Beat all tier 1 duelists 3 times each. Open at least five Blue Eyes White Dragon booster packs.

Weevil Underwood - Beat all tier 1 duelists 3 times each. Open at least five Black Skull Dragon booster packs.

Ghouls (selectable) - Beat Marik Ishtar once.

Ghouls (random) - Beat Bandit Keith, Joey Wheeler, Mai Valentine, and Pegasus seven times each.

Maximillion Pegasus - Beat all tier 2 duelists 5 times each. Have at least one of: Toon World, Blue-eyes Toon Dragon, and Toon Summoned Skull cards in your trunk.

Shadi - Beat Bakura, Yugi, Mai, Joey, and every tier 4 and tier 5 duelist 10 times each.

Bandit Keith - Beat all tier 2 duelists 5 times each. Have a Barrel Dragon card in your trunk and none in your deck

Faster Duel Speed:
When choosing a duelist, hold down L until the duel starts. The gameplay speed will be twice as fast.

Permanently Unlock Duelists:
Certain duelists have extra requirements to unlock. You must constantly meet these requirements, or they will vanish from the selection screen. If you defeat a duelist with special requirements ten times, they will be permanently selectable, regardless of their past requirements. For example, if you beat Joey ten times, you can then duel him even if you have a Jinzo card in your deck. Losses and ties don't count against your total wins.

Keep Thunder Nyan on the field:
Thunder Nyan Nyan is a powerful 1900 ATK Light element monster that is immediately destroyed if you have a non-Light element monster face-up on your side of the field. However, if you use Snatch Steal to take a monster from your opponent, it doesn't count as a monster on your side of the field, and therefore Nyan lives. This doesn't work with Change of Heart.

Spirit Reaper Glitch:
Spirit Reaper's effect states that it cannot be destroyed as a result of battle, making it a perfect shield, but if it is targeted by a magic, trap, or monster effect card, then it is destroyed immediately (this includes equip magic cards such as United We Stand or Axe of Despair). However, if you use an equip card on Spirit Reaper, the equip magic card will be destroyed, but Spirit Reaper will remain on the field.

Dark Ruler Ha Des Glitch:
If you attack and destroy a Slate Warrior with a Fiend type monster on your side of the field when you have a Dark Ruler Ha Des face up on your side of the field, the slate warrior will still weaken your monster, even though the effect of Dark Ruler Ha Des should prevent that from happening.

Fiber Jar + Solemn Wishes gitch:
Have Solemn Wishes active on your side of the field and then filp summon Fiber Jar activating its ability, when you go to draw your cards although Solemn Wishes isn't on the field you still gain 2500 life points!

Graverobber Card Exploit:
The Graverobber card states that you can take a spell card from your opponent's graveyard and it will only work if you activate the spell card you took during your turn, but you have to pay 2000 life points to activate it.

You can play the card you took without paying the 2000 life points by first placing it on the field and then activating it while its on the field. The game somehow forgets the card is in play because of Graverobber and normally plays out the card. You will however end up paying the 2000 life points if you play the card you took while its in your hand, so make sure you place the spell card on the field first before you play it so to avoid the 2000 life point penalty.