Abuse the range and reliability of your Q in the early game. The damage doesn't look very impressive, but starting at level 3 if you keep firing it off cooldown, it will add up (she works really well with poke adcs who can add their damage onto this harass)
Be wary about spending mana on your W during lane phase, it's only really efficient if you get value out of at least two of the four components (heal on you, heal on ally, shield on you, shield on ally)
Your power chords are hugely important. Early on you want to learn when you want that burst from a Q power chord, the slow from an E power chord when someone goes out of position, or the damage reduction from a W power chord when the enemy hard engages and you want to turn the fight around. The safe bet is Q in most cases early game although there are also a lot of times when you want E, as the game moves on the lines become a lot fuzzier.
In general you're probably better off building mostly AP (mixed in with athene's, zeke's and/or ardent censer) to keep up the harass throughout a game. But eventually you should learn to recognize when your team is working in such a way that they can benefit from the more cheerleader-ey items like locket and mikael's (also when you're good enough to remember to use your item actives)
As teams start to group your auras become more and more useful so make sure you are getting Q auras onto auto attackers, W auras onto allies when they're going to take damage, and E when your team is trying to rotate, chase, or run to save an extra couple seconds here and there.
And of course, remember when to use your ulti. If you can crescendo both enemy laners at level 6 then you probably just got your adc at least one kill, and if the enemy tries to engage on your team and don't realize your R is up, they will suddenly be having a really bad time.