Scott "Slurgi" Penick talks about never underestimating your opponent.
"Keep your minerals low", "Inject larva every 40 seconds", "Check the minimap", "Don't get supply capped", "Use your chronoboost", "Don't queue up units", "Trust what you've practiced", "Assume your opponent is going to make the most optimal play given his situation".
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Uh, wait... what was that last one?
There are a lot of mantras and mental checks that require methodical cyclic attention in order to maintain good game flow and optimize your chances of winning. It's a challenge to give them proper attention even in ideal circumstances, let alone when there's a medivac dropping blue flame hellions in your mineral line. Although not highlighted often, if you consider your opponent's best reaction for each major in-game decision, you will improve as a player much more rapidly. Sometimes this requires analysis outside of the game by watching your own replays so that you can respond instinctively to the same situation in a future game. Other times, you'll need to think on your feet.
So far, this has been really abstract; an example is in order. On most maps the current metagame for Protoss versus Zerg favors 3 gate sentry expand builds and nexus/forge expand builds. For this example let's assume that the latter was used and that few or no gateway troops have been produced and a solid wall has been formed such that cannons safely protect all buildings and probes near the expansion from melee attacks. Let's say the only way through to the main base is directly through the expansion mineral line and up a ramp. In these circumstances, it is never okay for the Zerg player to employ the "zergling run-by" strategy, wherein speed upgraded zerglings "run by" the expansion defences and into the main Protoss main base. Unfortunately high-level Zerg players still do this often under these circumstances, and it succeeds only if the Protoss player failed to block the zerglings' path using probes or by quickly constructing a building to wall-off the entrance to their main base. Sometimes the Protoss player fumbles and lose the game as a result, but as players improve, the "zergling run-by" under the circumstances outlined above will be less and less viable.
Even in professional games, this principle is violated. In GSL season 1, there were countless TvP games where the Terran player would push up the Protoss player's ramp, even when he knew the Protoss player had sentries. Somehow the Protoss players either failed to forcefield properly, forcefielded too late, or still kept zealots in sight range of the Terran force after forcefielding. Artosis criticized the games and chastised the players endlessly and there's very good reasoning behind his criticism. Most of the players that made these decisions were shortly eliminated and are now no-name players. What they did was a high-risk high-reward move that yields rewards only in the presence of their opponent making a critical mistake. Simply put, that's not a sustainable way to competitively play an RTS game.
Mr. Sentry, you failed at the only thing you’re good at
Artosis is the perfect spokesperson for this. In his dialog with Tasteless he often makes statements of the form "after action A, player X should do action C, since the best reaction from player Y is action B". As an example, if a Protoss player opens with voidrays in a PvZ game, it is reasonable to anticipate hydralisks as a natural reaction and preemptively begin tech to collosi.
Anecdotes aside, what I’m really getting at is this: if you practice consistently such that getting supply blocked is a thing of the past, your micro is good, and your macro rarely slips, pushing yourself to the next level can only be accomplished by imagining that every game you’re pitted against the smartest player in the world. At worst, you’ve overprepared against a lesser opponent, and at best, you will end up winning a game you wouldn’t have otherwise.
Slurgi is currently a 3700 masters player, often ranking top 20 of the Top 200