Here are some common mistakes. If you manage to avoid these, you'll improve your odds of winning significantly.
Not building enough workers
This is a very common novice mistake. Instead of building too few workers, make sure to build 22-30 workers per resource location. If you build an expansion, build the same number at the next location.
Too many workers on gas
You can only fit 3 workers on a vespene gas geyser before any additional workers become useless. If you think you have too many workers gathering from the same geyser, click on your refinery/assimilator/extractor to check.
Placing a headquarters far from resources
When placing your headquarters structure (command center, hatchery or nexus), place it as close to the minerals and gas as you're able to.
Be sure to Scout.
Make sure you're prepared for dark templar, banshees, and other cloaked units. Use abilities or build structures or units that can provide you with detection (such as ravens, overseers or observers).
Using Move instead of Attack-Move
If you use the move command to walk past enemy units, your army will ignore the enemy, likely taking damage in the process. If you use the attack-move command (default key ‘A’), your units will stop and engage any enemy units they come across. Learn more
If you only have one base, you may feel safer because you have fewer areas to defend, but you won’t generate nearly as many resources. If you want to create a large army, expand! Learn more about Expansions.
Not expanding before the resources at your main base are depleted
Losing access to resources - or any significant delay in your gathering of resources - can leave you at a catastrophic disadvantage against an opponent with a strong economy. Learn more about Expansions.
Gathering too many resources without spending them
If you‘ve gathered up a lot of resources, that means you're not spending them on creating a bigger army, building structures, or upgrading your units. Of course, it can be fun to gather a lot of resources and spend them all at once, but that won’t keep your army advancing or ensure that you’re more threatening to your opponent.
Queuing up too many units
At the beginning of the game, you’ll need to spend your resources on different types of purchases, such as workers, buildings, and combat troops. If you queue up too many combat troops or workers, you won't have money to build the other units or structures that you might need. During the early portion of the game, while your economy is still developing, only train the units that you absolutely need. If you find that you have multiple units queued up and not enough money to build or train something essential, click on the building producing those units, and cancel the training of one or more of them. If you're really desperate, you can even cancel a building that’s in the process of being built or an upgrade that's being researched.
Queuing up upgrades early
Do not queue multiple upgrades early on during a game - those resources will be better spent on units and buildings. However, later on in a game, it can be a handy and useful time-saver. Remember, you can always build multiple buildings of the same kind in order to research different upgrades simultaneously.
Allowing developing structures to be destroyed
If a building is unlikely to finish construction before it is destroyed by an enemy attack, cancel it - this will give you a 75% refund on the cost of the structure. To cancel a developing building, select it and hit ESC.
Failing to maintain sufficient supply
Make sure you keep pace with the construction of supply buildings (pylons & supply depots) or overlords. Otherwise, when you need to create a unit at a critical time, you won’t be able to.
Losing units along with a unit transport
When your transport vehicles (such as the terran medivac), are under attack, try to unload all of your units before the transport is destroyed.
Not using Alt
Hold down alt. That shows enemy and your own health bars. You can target units that are about to die or save your own damaged units.
Not watching a battle
It’s crucial to pay attention and control (‘micro-manage’) your units during a battle, unless you have teammates to help control them instead. Otherwise, your units will not perform to peak capacity. Perhaps they’ll target enemy units that aren’t a priority, use their abilities inefficiently. There are a few instances where you don't really need to watch your units (like tons of zerglings mauling a platoon of marines that clearly took a wrong turn somewhere) but, generally, it's best to stay in control of your army. It is possible, however to spend too much time watching battles and not enough time doing other things like macroing or controlling other units.
‘Teching’ too early
"Teching" means to advance along the tech tree by building structures that unlock specific units or upgrades. Let's say you're a terran player, and you try to reach battlecruisers too quickly. You might find that a bunch of low-level units like zealots, marines, or zerglings, constructed en masse, are able finish you off. Instead, wait until the proper time – when you have enough defenses and resources - to improve your units or unlock new options without risking disaster.
Not including every unit in a combat
There are a number of reasons to split up your army - perhaps you have new units heading to a battle, chasing enemy units or moving to reinforce a specific location. However, engaging an enemy when you’re outnumbered is dangerous - whenever possible, you should instead retreat, gather your split units together, and then fight.
Reliance on focused fire
Focusing fire (assigning all units in a combat to attack a single target) isn't a guaranteed game-winning strategy. Don’t desperately try to focus fire units to the point where they’re busy running around trying to reach enemy units rather than attacking. It’s better to attack a secondary target than nothing at all.
Attacking superior forces
Looking at an an enemy force should give you a good idea of whether or not you're going to be able to destroy it with your army. While you can't always know for sure, you should not attack armies that are obviously more powerful than your own. This knowledge will come from experience playing the game, but, in general, consider numerical superiority as well as specific unit types that might outflank yours. If a battle is not going your way, don’t forget to run unless there is no escape.
Attacking the enemy from a disadvantaged position
In general, don't attack your enemy if they have automated defenses to support them, or if they’re on high ground while you are on low ground. Avoid fighting the enemy if you don't have enough room to bring all of your forces to bear (for example, fighting in narrow areas). If you can fight your way up a ridge or destroy the defensive structures, you should - but take care so that you don't lose your army to a terrain disadvantage.
Unnecessarily destroying defensive structures
If you can move your army past attacking buildings (photon cannons, bunkers, spore crawlers and the like) and reach an enemy base out of range of these defenseive structures, you can save valuable time by doing so. Don't run past them if you're going to take too much damage in the process, of course.
Not upgrading units
Research upgrades - your units will benefit greatly from being upgraded. Armor, weapon, and shield upgrades are especially important.
Building defenses in unnecessary locations
Defenses tend to be maximally effective when they are placed near strategically valuable locations (such as your base, or right outside the enemy’s front line). It’s a common mistake to place defensive buildings outside of your base – these can be walked past or ignored. When you build defensive buildings, place them in your town, or at least prevent the enemy from walking around them by supplementing them with units or by walling your base off with other structures. There's no point in guarding a bunch of dirt.