Today's tip focuses on competition, and is aimed a bit more at the men, as they spend competitions with these attached to their backs:
The number is a couple's official identity for the duration of a competition, and as such it should be readable. Too bad that often numbers are made in flimsy cardstock, pierced with safety pins, easy to bend, easy to rip, and easy to sweat through.
One solution is to get some cheap clear page protectors. Bring those and scissors to the competition, and then trim a page protector to fit the number. You can pin the page protector to the guy's back, then slip the number inside. The number is now attached and protected. You can of course reuse the sleeve between competitions, assuming it doesn't get too mangled. Furthermore, if a guy has multiple numbers during an event, numbers will be easy to swap out since no repinning is needed.