There's no need to resort to plain rubber bands and damage your hair in the name of convenience - here's a quick and easy bun slide you can make using basic office supplies and one of those spare cards you're certain to find in your wallet.
I've worn this one at work all day, and it's kept my hair safely up and away from my 6-month-old's grabby fingers. Success!
Step 1: Find a Card
*I'd recommend against using credit cards for security reasons, but otherwise go to town.
Step 2: Punch Holes in Card
Make sure to punch (or drill) your holes about one hole diameter away from the card's edge for maximum strength. Sand off any rough bits left from your hole punching.
Note: if you've got lots of hair, punch the holes in opposite corners of the card - the resulting diagonal will hold more hair.
Step 3: Find a Pen or Pencil
I tested this with a pencil, but decided on a slightly thinner and pointier ball-point pen to avoid hair snags. Your mileage may vary - just use anything readily available that won't bleed ink on you.
Step 4: Pin Hair Up
Use one hand to hold the bun in place, then slip the card around it. Shove the pen (or pencil, or chopstick) through both holes, threading it underneath most of the bun as it passes under the card. This will bow the card, and pin the hair in your bun against it.
Nice work - you've done it! This clip should be more than enough to hold your hair in place all day.