Think about what theme you want to use for your tri-fold board. If you are working on a project about oak trees, think about how you could incorporate the image of a tree into your board. You might have a large tree in the center of your board with branches suspending the blocks of text for your project.
Add additional cardboard pieces to your board to make it stand out from the rest. Think outside the box; instead of decorating the board, think about changing the shape of the board itself. For example, if you have a tree theme, you might cut out a large pieces of cardboard in a half circle and glue it to the back of the middle section of your board to simulate the look of a bunch of leaves on top of the trunk of the tree. You could then cut out green leaves from construction paper and staple or glue them onto the cardboard hemisphere.
Add smaller decorations that add on to your main theme. These could be drawings of animals, small plants, people, tools or anything else that fits with your theme. People are especially drawn to the human face, so think about how you could incorporate pictures of people's faces into your poster. Going with the tree example, you could have squirrels climbing up the tree trunk, a mouse in the grass below, a monkey hanging off a tree branch pointing towards your hypothesis text block, or a picture of the face of a politician, scientist or book author who is helping to fight deforestation.