How to keep fredh
You can just keep them covered to avoid the top from drying out. Also filling should not be too moist to keep bottoms from becoming soggy.
If you're using a moist filling. You can coat the bread with a thin layer of soft butter before filling to store them. You might even want to chill that a bit first before adding the filling so the butter sets back up.
Anything more than a couple of hours in advance is gonna be soggy or stale. Just ask your kids how great their lunches are when made the night before.