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I assume you mean "bun" and not "bum". I there are short hairs in the back that don't fit into the bun I just use a bobby pin to pin them down. It's that easy.
Sloka
I use Bobby pins and some hairspray
Elaya
use bobby pins and/or hairspray. always does the trick. I recommend both tho
MsBittner
You use inconspicuous barrettes, clips, or bobby pins to hold them in place.
Just A Nobody
Bobby pin them up. :)