How can you make the materials that you're using a bit stronger? (Sometimes it makes sense to use different materials altogether. Don't make a skateboard out of paper. Use wood!) But what can you do if you don't want to use completely different materials?

You can varnish it, or cover it in sticky back plastic, or if it's paper, add wallpaper paste (yes, papier-mâché - which is much stronger than paper!) Or make a frame to go around it.