Everytime I press D + LMB I see the pencil as cursor and the indication that I'm about to draw something, but every time I draw a line, the line disappears.
Check to make sure you don't have the [Grease Pencil layers] and [Grease Pencil Colors] set to invisible (eye icon). I spent an hour trying to figure out why, but forgot that I turned them off.
For the new players: N to open the right hand menu. Uncollapse the ▼ ☑Grease Pencil and ▼Grease Pencil Colors Look to make sure the eyes are visible for both layers. Also try creating a new Grease Pencil layer and seeing if that fixes any issues.
I'm using a 3D mouse for zooming, so my grease pencil drawing planes are created in weird angles. As a quick fix, I set Scale Y to $0$.
When I then draw a stroke, it instantly shrinks to an invisible point at the drawing plane origin.
This does not happen, if I set Scale Y to $0.001$
Just found a better way to fix the grease pencil drawing plane angle. Change the option at the top of the drawing window from View to Front (X-Z).