I bought a helmet online for $100. Once it arrived I gave the helmet a big wash to get rid of any dirt/grime, disassembled the helmet taking off the visor, mouth piece and internal padding.
Finally, sanding the helmet down and spraying a base white coat all over the helmet.
Initially I thought of doing some crazy graphics on the helmet, but I thought that if there was too much going on, on the helmet then what I'm actually trying to show will be muddled and confused. I settled on the most basic style possible, black text with a white background with some accents to give the helmet some design aspect.