This trick works best with Marth and Caeda (after promoting her to Dracoknight). Any unit should be able to pull this off as long as you're willing to devote the time.

Required: Any Chapter with an Arena and Venders, An available savepoint, At least one unit that can use healing staffs (two if early in the game), Enough money to make at least one Arena wager.

Clear the map, but don't seize the endpoint! Move your units over to the area with the Arena/Venders, then save the game. Enter the arena once or twice (depending on unit strength). Heal your unit with the healer(s) and end the turn. You get experience as well as cash at the end of each battle and your healers gain exp from healing. With the venders right there, you can buy all the staffs you need to keep it up as long as you like. You can repeat this ad infinitum.