I think my biggest piece of advice to a new AST is: get a feel for your party. If you're like me and you do a lot of PUG groups via DF, you'll run the gamut on different players and as a result you'll have to change your play style. There is some really great advice in here I won't duplicate but my tip is this: just keep practicing and learn who you're running with. Sometimes you'll need to adjust your buffs/debuffs depending on player skill, speed of the run and of course luck of the draw.

For example, maybe you've got a really bad group so you throw an arrow on yourself for faster casts instead of the DPS, to save your Lightspeed CD for a boss. Or perhaps your cohealer is blasting through their MP for some reason, so instead of laying it down on royal road, you give it to them. Playing an AST to me is learning to play smartly to the group you're given. And that comes with continued regular practice, which includes mistakes.

Good luck!