Numbers are not a regular thing in gymnastics. We only really see them at big competitions, like Easterns/Westerns and National Championships. In cycling most guys get pretty good at pinning their numbers on. I think it is more from necessity of not making it into a parachute when you are riding around. Since I have had to pin on a few hundred numbers over the last few years I have gotten pretty good at doing it for my girl's in gymnastics competitions. Here are the do's and don'ts when you want to get that number looking good, and stay looking good all competition long.
1) Crumple that number up and do it a bunch of times. This adds thousands of little creases that will make the number move more like cloth. Once you flatten it out again and pin the number on people wont ever notice.
2) Get the number centered. The best way to do that is place the number where you want by hand, then put one pin in the center to hold it while you do the first two corners.
3) If you are pinning a gymnast DON'T HAVE THEM BEND OVER! Pin the number on while they are standing straight up and make the number a taught as possible. Remember a leotard stretches and it will stretch when the gymnast moves. The paper used for athletic events is very tough and generally wont tear. (except at the holes made in the number, but more on that later)
4) DONT USE THE HOLES IN THE NUMBERS!!!! At my invitational we use numbers to help the photo people with orders, and to help people know which gymnast is which. I told our parent organization to forget the holes as they always tear and they actually cost you more money. They were afraid people wouldn't know how to pin on the numbers then........I know we are not rocket scientists but I hope a person could realize you can push a safety pin through a piece of paper. Instead, when pinning on the numbers, pinch up a corner with some of the leotard fabric behind it and then pin through all of it. You would then have two pin holes at each corner. With this method you will have a hard time ripping through the number, even if you wiggle around on it.
I know this is kind of a stupid thing, but it is something I care about for my kids. When you never have a number on you don't want it to be flopping around and distracting your athlete in the most inappropriate time. When all else it equal, sometimes it is the little things that make the difference.