You need to use at least the 3.3x table + 1nm/15kts for a still air descent, unless you're at Mmo/Vmo and/or very light.
Flight with unreliable airspeed: keep around 2.5degs between the FPV and the "airplane symbol" and you'll stabilise at a safe speed for whatever configuration you're in. Refine using the QRH figures once under control.
The FMC can sometimes come out with some very strange recommendations for cruising levels, often very low or very high. Once you're into the mid-to-late 30s, the payoffs for climbing are not that great, especially in the last few hours of flight.
Vnav descents at high altitude need watching if there's a significant change of wind component on the way down.
sit for max 20 min then go for a walk or stretch.
Probably the most important one!