Every unit unlocks jobs on its own. When I say "Knight Lv. 3," I mean that unit has to reach job level 3 as a Knight. Another unit's levels don't count for it, which is the most common question I get by e-mail.

Squire and Chemist are open from the start. Everything else grows out of those two, in two branches each, and then the advanced jobs need levels from more than one branch.

• Everyone through Monk early. The road is short, Squire to Knight to Monk, and a Monk is strong enough to carry a unit through the middle of Chapter 1 and into Chapter 2. Even my mages spent time as Monks, to learn Chakra and Counter.
• Pick your Calculator early. That road is long: Priest, Wizard, Time Mage and Oracle, all four to fairly high levels. Choose one unit with high Faith and start it down the Chemist side right away. Don't spread the work across three units and end up with none.
• Mime is a project. Level 8 in Squire and Chemist means going back to the two weakest jobs in the game for a long, long time. I've only done it once, with one unit, on my second game, by keeping her in Squire through a lot of random battles. It was worth it, once.
• Remember spillover. Units sitting in a job they don't need can still be gaining JP in the jobs everyone else is using. If four of your units are Wizards, the fifth is getting Wizard JP too.
• Random battles are your friend. If you're stuck on a story battle, walk back and forth between towns on the map and fight what you meet. It's slow, but every random battle is JP.