I found some instructions on how to make a Valentines heart latte courtesy of Food Service Warehouse:
Heart
The heart is a sweet way to show your latte drinkers a little love. The design is simple but affords room for many different variations.
Pour the milk into the center of the cup until the milk begins to pool in the center.
Start moving the milk pitcher from side to side in a vigorous motion t
Move the pitcher towards the edge of the cup more slowly, for zigzagging milk that is fairly close together.
When you reach the edge of the cup, move the pitcher to the opposite edge of the cup, effectively cutting the zigzag pattern in half and creating a heart.
The image here is from rate my rosetta which has loads of great tips and ideas for making your own latte art.