I'm not sure how much of an exploit or bug this is, and how many people might already know about it, however I'll just share it with those who don't know.
The trick is simple, and can be done anytime, anywhere, although the most effective place is when you are recovering HP. All you have to do is simply switch to a robe armor, sit down and start recovering HP and MP (the robe increases your max MP!

). When you're done, you switch back to your armor, and whaddayaknow, you have quite a bit more MP than you're supposed to have, meaning you can do more Mortal Blows or whatever all those fighter skills are called. This can also be done if you have armors that give HP bonus. You just have to remember to let it fully recharge your HP when you switch back.