Last week I went to Tuscany with Bombay Sapphire to see a demonstration of juniper berry harvesting. I'll try to post more information in a little bit, but for now here's how they do it:
Using a flat, round basket and a short hard stick, they whack the juniper bush branches so that the berries fall off. They want only the blue, ripe berries and not the green, unripe ones that will be ready the following year. Thus they must hit the bush with a certain force that removes one ripeness of berry and not the other.