Best approach to making a loaf of bread stale
Don't use a paper bag, it actually helps the bread keep somewhat longer.
Your best approach is to put it in the fridge. This is the temperature at which the starch crystalizes at the highest speed.
Update, shamelessly stealing from Tristan: If you cut it up first, you will speed up the process too. The more surface you have exposed to airflow, the better.
You'll still need to give it time, but it's maybe half the time it needs at room temperature.