I don't know if anyone else has discovered this yet or not, but last night I discovered a ridiculously easy way to level up moogles and Nu Mou. If you have one of these combinations of party members, you can make this trick work:
MoogleX2
Nu MouX2
Moogle and Nu Mou
First, the moogle(s) must know either Smile (Juggler A-ability) or Quicken (Time mage A-ability) and the Nu Mou must know Quicken.
All you have to do is enter an engagement with at least two of these guys, and when it becomes the turn of one of them, use your Smile/Quicken on the other one. Then, on the other one's turn, use Smile/Quicken back on the first. If you shift back and forth like this for many turns, the exp really piles up. This is an insanely easy way to level up characters, and I guess that's why they didn't make a quicken type ability for all races, lol. Keep in mind that for lower-level characters, the exp will be substantially greater than for higher-level users. But if both users have the same level, they will each get 10exp per turn.
Another thing you ought to consider is what class you want your character to level up in. I levelled up two Mog Knights like this, and got a couple of high-attack moogles out of it. Keep your stats in mind when you level up. My speed, magic, etc. were high on my moogles, so I levelled mine up for attack, but that approach isn't necessarily best in every case. You may want your moogles/nu mou to be faster, have higher magical attack, etc. so be sure the character is in a class that gains high stats in the area you want. If you're not sure which class is best, check out one of the faqs at www.gamefaqs.com under the final fantasy tactics advance page.
Let me know if this technique helps anybody. Or if you think I've just cheapened the Moogles and Nu Mou into little exp-grubbing maniacs, you're free to tell me that, too. :P