I was taught at a third wave coffee shop that required all the baristas to do latte art because 90% of our drinks were made for here in mugs. It's a lot harder to do it with the mastrenas and the pitchers at starbucks because they're so huge and not designed for latte art, but quantity.
Dont over aerate, but make sure that when you're steaming there are minimal bubbles and you get a good swirl. If you have time, or are just practicing, hold the pitcher the entire time, dont set it down. When you pull shots, angle the cup so that the espresso slides down the side, that way there arent any air bubbles and you have a smooth canvas to work with.
When your milk is done steaming, tap the pitcher against the counter if there are any air bubbles, and swirl the milk inside the pitcher to see how good your foam is. You want the fresh paint sheen they always talk about, but this also shows you the consistency of your milk. When you swirl it you can see if it's too foamy or not foamy enough.

When your pour, go slow and in circular motions. About 2 or 3 inches from the top of the cup, pull up the pitcher and stop pouring. Then move the pitcher closer to the cup and push the foam that is sitting on top of the milk in the pitcher.
For leaves, I do a wrist wiggle and move the pitcher back, then pull up and push a line through to the bottom of the leaf.
For hearts, you push the foam out slowly and do not move your pitcher, the foam will start to circulate and you can just push through to bottom of the heart to finish.

I am by no means an expert, but this is what I do and most of the time it works. However, starbucks equipment is not designed for this, so there are a lot of times it simply doesnt work so dont be too hard on yourself