The easiest way to to do this is to go to Falkreath.

Go to the barracks and to the jail (down the stairs).

Pickpoket a guard down there and take the key.

Open the cell and stuff your items in a sock thing.

Get caught twice. The first time get the key and your armor.

Leave the cell, go into the barracks and talk to a guard.

He'll say wait... I know you and submit to jail, then serve your time.