what is the best way to loosen up an over under shotgun action that is a little hard to break open. can you just sand down the action's sides a little.

use it! If you sand anything down its just going to wear out faster. They break in over time.

Yup. Go shoot the poo poo out of it.

Make sure you have a good grease on the hinge pin and go shoot it.

Can you sit at the house and work it back and forth? That'll save on ammo!

Do not sand the side of the action. That isn't what causes it to be tight. The lug under the forearm is too tight. It can be filed very small amounts at a time to get the feel you want. Just get a fine file and work the back flat side of the barrel lug and replace the forearm until you get it how you want it. It doesn't take much so do little bits at a time.
After you get it where you like it, get a bottle of birchwood casey cold blue and reblue the lug where you filed it.

thanks for the help

Let me have it for a couple of seasons to break it in, get some character built in it, and when I return it you will not believe it is the same gun.