This appears to answered by a few… but here is another one.
I use fabric grade double sided tape and place it where I want it on my jersey.
I then place my beloved RaceDots without worrying about the Race number flopping about.
Found this helps keep the Number flat and aero on my back!
When done, I remove the number and use baby powder on the tape which covers up the glue.
That then gets pressed between a pile of heavy books for a while to get it back to flat… and then finally joins the Wall of Numbers!