Get Cid:
Beat all 300 missions.

Get Ritz:
Beat all story missions. You'll now get the 'Mortal Snow' mission at the pub. Beat it to get Ritz.

Get Shara:
Beat the game, then clear A Maiden's Cry mission. Now enter a town.

Get Ezel:
Gossip with Ezel at his card shop until the Reconciliation mission appears in a pub.

Get Babus:
Beat the game, then clear Left Behind mission. Then enter a town and clear With Babus mission.

Get Lini the Mog Knight:
On the Tower Ruins mission use 'The Hero Gaol.' When you finish the battle a recruitment box should appear for Lini.

Get Litlevili:
Win the palace Clan Tournament twice.

Recruiting clan members:
If your clan drops to 5 members or less, a 'Dispatch' mission will appear in a pub. It won't count towards your 300 missions list. The reward is a new member depending on the month.
Kingmoon = Human
Madmoon = Bangaa
Sagemoon = Nu Mou
Huntmoon = Veria
Bardmoon = Moogle

Expert Missions:
Beat the game once, let the credits roll and save over your last saved game file.When you load that file again (it will now have a star next to it), you'll be able to access new missions from bartenders at the pub.

Easy XP:
Buy 99 potions and go into battle. Keep using potions on yourself; you get roughly 10 XP each time.