To my knowledge, you have to use items to heal yourself out of combat. All you need to do is just wait until your next fight and then heal up there. Take out the most dangerous monsters until you have just one left and keep skipping turns and blocking while the healer tops up everyones' health. The only time you should really need to heal out of combat is if you are about to face a boss or if the character's health is dangerously low.
Edit: The L3 button skips turns, by the way. I didn't find this out until later stages of the game but it comes in helpful sometimes as the battle screens can get quite large. If one monster is left on the opposite side of the screen, you can group your characters up and make them come to you.