Just tell me the ways.
Here's what I have thought of real quick:
~ Gain Every Turn in battle
~ Gain Every Time you defend in battle[/list]
Ways to Gain
~ Based Off Actor, class, weapon, armor
~ Random
~ Use of skills on a certain actor
Let me know if theres any ideas. Of course, you will always have the option of
<actor>.gain_skill_mana('name', number)