I've found the best way is to do FATEs solo while queuing for dungeons. Yes, the initial roulette bonus is the big one, but even after that you get a lot more exp doing dungeons than FATEs for the comparable time spent, at least at low levels. I can plow through a dungeon in about 20 minutes and get 75K or so for a level 30ish job, and that's without the bonus. There aren't FATEs for that level that are going to give that kind of reward in that kind of time.