Yu-Gi-Oh cards and other trading cards can sometimes bend or crease over time. Cards can become creased or bent when they are handled roughly or not stored properly. In addition, foil cards often bend or warp, especially in high temperatures. Creases can be repaired somewhat, although they decrease a card's value and can make it unsuitable for tournaments. Fortunately, though, bent and creased cards can be straightened again with a little care and patience.
Clear a flat, hard surface such as a table or work bench. Make sure it's dry and very clean; small amounts of grit or dirt can press marks into the surface of your cards.
Lay your Yu-Gi-Oh! cards on the flat surface. Place them in a single layer, without stacking or overlapping them. If your card has a crease, gently fold it back in the opposite direction. If it is bent or warped, place it with the "hump" of the curve facing up.
Place a stack of heavy books, such as the Bible or science textbooks, on top of the Yu-Gi-Oh! cards. Lower the books gently so you don't disturb the cards. Placing the books down too quickly can cause the cards to shift, perhaps overlapping each other, which will prevent them from flattening properly.
Alternatively, you can place one or two Yu-Gi-Oh! cards within the pages of a particularly heavy book, then close the book and lay it flat. Don't forget that the cards are in the book, though! They could end up getting lost, or fall out and become even more bent.
Place the flattened Yu-Gi-Oh! cards into card sleeves (deck protectors) to protect them from further damage. Store them in card boxes or a binder to prevent them from sliding around and bending or creasing further.
Make sure you have the right size card sleeves for your Yu-Gi-Oh! cards. Storing cards in deck protectors that are too big or too small can cause them to bend. Avoid extreme cold, sudden temperature changes, and moisture, which can cause Yu-Gi-Oh! cards to bend. Store them in a cool, dry place. These steps can be used to flatten other bent or creased trading cards, such as Magic the Gathering cards and baseball cards.
Do not attempt to iron Yu-Gi-Oh! cards! The heat and steam of an iron will cause further damage.