-For easy yen, go to one of the fishing spots. The later in the game you find it, the better. Start fishing. For the first four or five fish, catch the smallest fish to save HP. Afterward, get the big ones and they will be rare since you have been fishing for awhile.
-For maximum yen in battle, get the Thieves Glove and steal an item from every monster. If it is a Sake, use it to speed up the battle and get a bonus easier or sell it afterwards. If it is a Bone, do not sell or use it unless you need it. If it is treasure, sell it after the battle.