Always have 99 potions in store when you go into a dungeon. It's a lot better to waste them then it is to waste your magic.
If you get stuck in a dungeon, memo save. Walk a few more steps and memo save again. Once you get in a battle, restart. Load up that memo save and walk some more. Memo save. Repeat and rinse to get out of dungeons easily.
Use magic sparingly.
Save a lot.
If you train, do it where there are a lot tougher monster to what you're used to.
Always keep a tent in your inventory.
Don't overkill the enemy. If you got a lot a water based enemies, don't lay bolt 3 into them when bolt 2 will do well. This rule holds true in dungeons were people will feel it's cool to overkill the enemy with super-powerful magic. Later, when you need that magic, you won't have it.
When ever there's a treasure chest by itself. Assume that there's also a tough monster in front of it, too.