First i will list a method to practice playing so, that one only creates a single unit when playing the game.
I have selected two units for this practice, for Terran i have selected Widow Mine but allowing also use of ealry marines and dropships and for Protoss i have selected Stalker but allowing use of Warp Prism and Observer.
Reason why i have selected these units are that i think how one uses them will greatly enhance the performance on each race.
Reason why other units are not allowed is to force the player to be creative around the unit, to create strategies and solve problems with them that they would not do otherwise. When starting to play normally, the perspective might shift and the player might find new angles for said units. Who knows, maybe next time you might warp in Stalkers instead of Zealots on the opponents main, shoot stuff and blink them away to join the main army instead of sacrificing them. Maybe next time the opponent have 60+ Banelings you have similar supply of Widow Mines yourself.
The control on said units will also surely improve and if you actually manage to win some serious games using only Widow Mines, i call that a feat in itself.
The second method is for Zerg and is focused on scouting. The player should focus on having vision on all expansions at all times. Be it with Zergling, overlord or so. The purpose is to make this routine so that the player do not have to think about this and will always have vision on all mineral locations on the map at all times in each game that does not end at early stages.
If someone wants to check this being used by a player, Larva does this in every single game in ASL season 10 round of 24.
If someone wants to check player using a single unit, using only stalker can be seen on AlphaGo vs Mana series. This is the unit the AI have chosen to be mass produced against all threats as it have the most potential.
Hope these ideas will help someone improve their performance.
I will give one additional challenge on this post. Upgrade mech and structures, create only mech units and only liberator from air. Always accompany an army with several SCV to build and repair. Build offensive wall ins to disable flanking of sieged positions by starting several depots at once. Do this immediately after taking a position even if the position would be temporary but still do this every time. Always try to repair each damaged unit and structure all the time, even in middle of the fight. Use liberators also versus air together with cyclone + turret.
Focus of this practice is to learn to support your mechanized army with SCV as an offensive support unit and to practice siege positions. Always leave some room (and preferably buildings) between siege tanks, mines and so. Overproduce SCV, try to have more than 80 as fast as possible, keep producing and sending them as they fall in front line.
Reason to produce only liberators is to notice their usefulness and learn to use them in all situations. Try to have range and at least 10 of them as fast as possible, use the first ones against economy and try not to lose them, but bring them to base for repair. If opponent lacks air units, use few of them constantly on economy. Avoid sending hellions, send liberators. Support your main army with hellions instead, avoid sacrificing them. Do not send mine drops or produce dropships, use only liberators as a separate force outside main army.
Reason to constantly focus on SCV is to notice their versatility over other similar units.
The real game alteration of this build will use at least raven and if depends then viking. Main focus should still be in liberators and producing 2 at the same time.
Spells that should be cast every single time in each fight, shield and anti armor missile. Spells that should be cast against protoss every time EMP. Unit which makes the biggest difference between amount of 0 versus 1 is Disruptor. These things do not depend on strategy but are mandatory.
The final practice is to learn to cast spells in each engagement.
If one thrives to be efficient, these things are not to be overlooked.
The above mentioned mech build fares better than the rest without anti armor missile or emp and does not necessarily need them, there is lot to do and practice already.
Before anyone asks, i do not myself play this game so i am not shifted on any current meta. I believe these things have always been so.