Start by having a background layer and an object layer (logo or text), To make the long flat shadow, make a selection out of the object by pressing Ctrl (Mac: Ctrl) and clicking on the layer thumbnail in the Layers panel.
Create a new layer below the object layer by holding Ctrl (Mac: Ctrl) and clicking on the new layer icon. Fill the selection with your shadow color, and with the Move tool active, hold Alt (Mac: Option) and press the down arrow key, then the right arrow key. Repeat this step as many times as you need to make the shadow longer. When you’re done making the shadow you can press Ctrl D (Mac: Ctrl D) to deselect.
You can also use this technique to create 3D looking text.