It's one of the oldest challenges in aviation. You have to learn to associate an action with every important clearance. A clearance can be a trigger to write something or select something. Use a pen (remember those?). I personally use the MCDU scratchpad. "Taxi A, B, C1 to and hold short of 27L" becomes A/B/C1//27L. When I'm lining up, I will ad a /LU. Takeoff clearance is definitely the selection of the "Takeoff" light on Airbus. On Boeing, I don't know what you might use, but for gods sake, choose one!