To make a long black, you pour espresso on top of hot water. There are a variety of reasons for this, but a primary one is to avoid dissipating crema by pouring hot water over it (which is an americano).
But how do you preserve the crema when pouring it into the water? To me, the act of pouring appears to break the up crema entirely.
This link from Seasoned Advice might help you. The accepted answer recommends to use an espresso brew pitcher and how to use it. The answer suggests two main things:
See more information at that link. It suggests that if you pour it in too fast, you ruin the foam. Too slow and the foam gets stuck. Practice sounds important here.
I run two coffee shops and what i have found is there are two ways of keeping the crema nice when pouring it into hot water.
You extract the shot with the tip of the portafilter in the hot water(this gives the best result as any fall even short distances breaks the crema)
When you poor the shot of espresso into the drink you poor it SLOWLY down the SIDE of the cup.(I do this even when extracting a shot for a cappuccino i let the portafilter poor the coffee down the side and not just let it fall to the bottem)
