I was able to level my second job from 50 to 70 ridiculously quickly by just doing Main Scenario Roulette and 50/60 Roulette, with some sidequests and FATEs in between. It's a lot of experience for those two roulettes, and plenty of poetics tomestones to have a full set of Augmented Shire gear just waiting for you to hit 60. And you'll never be undergeared during that 60-70 push; that Augmented Shire gear will always be great for every dungeon you're doing in those roulettes.

This strategy could probably be easily adapted for a first job by doing MSQ instead of sidequests. This will also give you better gear from 51 up for free. It might not be quite as quick without the Armoury Bonus, but I didn't notice much of a dropoff in experience gained at all after 60 when that Armoury Bonus went from +100% to +50%.