Posters, commonly made on poster board for school projects and science fairs, are actually considered enough of a professional tool in professional science communities that The Oceanographic Society has an entire booklet dedicated to creating good poster presentations. While the scientific community often uses large posters to get their points across, many of the same presentation concepts apply to both posters and poster boards, including making the poster visible. You do not need a PhD in Physics to make sure your board stands up straight and can easily be seen.
Attach your board to a straight surface to give it solid support. Options for attaching your board include gluing or taping it, using magnets or wrapping rubber bands around it and the object it's against. Look for a surface where you will be using the board, such as a wall or door. If you need to move your poster board, attach it to something that is easily movable, such as a chalkboard on wheels.
Lean the board up against another surface with a slight angle to support it. Though you can lean it at a low spot like the back of a desk or the wall, many classrooms contain places to rest the poster board that are already elevated, such as the seat and backrest of a chair, a window ledge or a chalkboard. If you are in a school you may also have access to items such as an easel or a large foam triangle. If your board is too flimsy to stand up straight while leaning, place a piece of material, such as a piece of cardboard or a light piece of wood behind it on the same slanted angle to provide a stiff "backbone."
Use your environment and find something to hang your board from, such as hooks or nails, the ceiling, the edge of a table, or any other convenient place. There are a variety of options for creating a hanging your poster board, including punching two holes at the top and stringing thread through the holes; attaching a binder clip to the top of the board; and if it's sturdy enough to not bend, punching a hole in the middle of the top of the board.
Using an audience member to hold your board is a way to have both audience participation and a straight board. This method is particularly useful if you have any images or text that the audience should see up close, as the person holding the poster board can take it around the room. You may also hold your own board as you present, holding it with one hand and using the other to point or gesticulate, or bring a friend along to hold it for you.