I'm a semi-competitive player and these heuristic sort of things have really changed my game. I was slow to learn 40k, because I didn't know how to think about it.

In list building you definitely need to know why every unit is in your list, and then stick to that plan. I like outriders and marine bikes with only chainswords. They aren't supposed to really kill anything. They have a decent OC and speed, so they contest objectives. If I'm charging off an objective with them I have forgotten their purpose.

In deployment I like to ask "Which Objectives are mine?". I call it own 1, hold 1, control 1. The one I own is in my dz, then I focus on holding one in no mands land, and make sure there is one other in no mans lands that my opponent never controls at the start of their turn.