Ask someone to do it once, then leave the pins in your jersey for the rest of the year.
**
On the other side of this conversation: how to stop getting people to ask you to pin their numbers. You would think that stabbing people repeatedly with "safety" pins would do the trick, but apparently not. Some pointers to get out of this:
* use the holes, rip at least 1 of the corners
* pin the lower corner far too deep into the arm pit area so that the remainder of warm-up they are irritated by metal rubbing there
* start breathing heavily while doing it
* stab them on the last pin. Always the last pin. As they walk away say "oh wait let me fix that"
* say things like "jesus christ, if I stab you I wonder if you'll even feel it with all this lard on your ass"
* at the end, make them hold the position on top of the chair as if you are an artist admiring their work. After a full minute say "no this won't do" and start it all over again
If they use glue, and give you the number with glue, make sure you drop it on the ground first. Then ask for the glue can and spray it on everyone in a 23 foot radius, then put it on upside down.
HTH