Use the "Q" Hotkey to move and fire at the same time or to make your units go to different locations one after another.
Building on Low Power is 50% slower.
A full harvester of green tiberium is worth 700$, a harvester of blue tiberium 1000$.
As long as the respective production building is alive, you don't need to keep technology buildings. You can sell them to get money and a couple of infantry back, and keep the building options.
A disruptor does more damage when shooting diagonally through buildings. If angled correctly, it can one-shot refineries and warfactories by "force firing" to the opposite corner.
By using "alt+click" harvesters and other vehicles can crush infantries, can even be combined with "Q".
Making a harvester in the warfactory costs 1400$, building a refinery and selling it afterwards costs only 1000$ and even leaves additional 5 infantry behind.
After giving your carryall the command to land on a unit, you can order that unit to fire at a target, so it will keep firing until the carryall lands. It will also start shooting right after being released again.
Orca-Bombers tend to waste a lot of bombs when being told to attack a building. You can "powerbomb" by flying them close to the building first, and then attacking it.
Use "ctrl+click" to force fire. Can be used on the ground to extend the range of disc men.
Select a production building and "alt+click" anywhere on the map to set a rally point for units produced in that building.
Three standing light infantries will kill a single light infantry walking by.
If you want to protect your base from disruptor drops with RPGs, place them a couple of cells apart, so that they cover each others blind spots.
Five banshees can one-shot a disruptor.
Ten banshees can destroy a main building (construction yard/warfactory/refinery) in one shot.
3 engineers can steal an opponents building when multi-engineer is on (you cannot steal an emp)
Put your units on guard by pressing G to allow them to attack units in the area
You can sell units by placing them on your, or your allies, service pad. Wait until the green dollar sign appears and click to sell the unit
Place a building underneathe a landing enemy carryall to temporarily prevent it from landing
Harvesters full of blue tiberium can be used as a 'harvest bomb' with enough damage to destroy a refinery or a war factory
Control your harvesters to eat the tiberium nearest to the refinery for faster consumption and money
The colours 'gold' and 'red' and 'pink' and 'purple' will result in opposite locations on the map Terrace. Pink will ALWAYS be bottom left or bottom right
Quick hotkey cheat-sheet.:     R=repair,   W=set waypoint,   P=power down,   F= Follow,   V=view radar event,   G= guard,   D=deploy,   Q=que unit,   T=type of unit,   E= select all units,   H=view home,   X=units scatter,   S=units stop,   A=ally your teammate, etc.....
If you place a devil's tongue right next to a building, the flame attack will do more damage to that building.
If you hijack an enemy's Subterranean APC when it is occupied by a Cyborg Commando, your enemy cannot build another "CC", until that sub is destroyed.
To get more money out of each tiberium field try eating the tiberium directly around the tib-tree FIRST because it will regenerate quickly.
There are many different "build orders" that a player can choose from, test new build orders to see how they hold up to other players' strategies.
If you are facing a NOD opponent you do not need to space out your buildings as much as when facing a GDI opponent, but you might want a sensory array and an EMP for subterranean units.
You can place defensive units on guard (G) to defend your base automatically while you are busy attacking your opponent.
By accessing the interface controls within the game, you can bookmark the enemy's base to a single keyboard command, which can save time scrolling across the map to see their base.
When you place component towers you can save money and build time by leaving them empty. When you are attacked you can quickly place a SAM,RPG, or Vulcan to defend your base.
NOD is usually better at mod maps because you can bunker down with SAMS and artillery near the tiberium. The solution to defeating a massive bunk is to outbuild and use a EMP.
In a team game, you can put engineers into a teammate's sub APC   ...and...  After you are defeated, your engineers will still be alive so you can grab your teammate's MCV in secrecy, hopefully.
A ghost can kill a carryall (carrying a mk2 or other unit) in one shot if you shoot the carryall right as it lands. You have to be sneaky to pull it off, try hiding behind buildings or under trees.
Stealth tanks do not shoot at units passing by unless you command them to, unlike many other units, so they are not very effective as defensive units.
Westwood maps and modded maps require completely different build strategies depending on the map's terrain, amount of tiberium, the number of players in the game, and many other things.
To automatically protect yourself from NOD's engineer rush you can put infantry around a building, use lasers at the corners, or use a waypointed bomber defense. 
If you have 2 harvesters and 1 refinery, you can fill the first harvester up to 3 bars and send it back to the refinery so that by the time it's done, the second harvester is on its way back, this  can reduce harvester wait time temporarily.
Waypoints are very useful in TS. They can be used to auto-engineer buildings, set up a bomber defense (GDI), or to use harpies to individually auto-shoot infantry.
You can automatically reload your planes by placing one of your waypoints  (in a waypoint loop) on  a helipad.
You can follow harvesters (or any other unit) with protection units using the F key.
A sneaky way to kill your opponent's harvesters is to hijack it first then kill it with a Cyborg Commando. After you steal the "harv", your enemy won't be warned when you start killing it.
It takes 7 shots from a titan to kill a disruptor, this means that 3 titans can kill a normal "diz" attack, and 4 titans can kill a one-shot "diz" attack.
Subterannean units can surface directly under an engineer to kill it. This is done with the ALT button and can be a great way to clear the way for a banshee attack.
Pavement can be useful to protect your engineers from underground units "ALTing" up under them, but it is expensive, especially on a westwood map like terraces.
In a team game, if you are broke and can't afford a tiberium refinery,  build  a silo and send your harvester to your teammate's refinery. This will allow you to make money without having your own refinery. (Note: your teammate must manually stop his harvesters for this to work or else they will block the refinery from you using it)
If you want to become elite and beat pros, you better get good at simultaneously managing attack, defense, out-building, and harvester control.
Try to think of every engagement in terms of how much money or build time you are trading with your opponent. Was that diz attack really worth it if they sold the refinery and your diz died?
3 bombers can take out a refinery if you attack it diagonally.
3 titans can kill a 1-shot disruptor ONLY if you CNTRL fire on the carryall as it is in flight, then switch to shooting the disruptor after it has landed. (hard to pull off)
You can kill a full health MCV base with a multi-missile by placing the "missile-pointer" just to the top left of the middle fan on top of the MCV, you must also time the missile so that the pointer points to your missile silo when you launch it.
If you have scouted around an enemy but not in his base, you can use the "S" button to stop a sub unit under their base. This is an example of what is known as "cross-scouting".
You can hijack an enemy's harvester full of blue tiberium and use it to bomb your opponent. Try to steal a full one that is already heading back to their refinery.
You can draw out your enemy's defensive units by attacking their harvesters, which will leave their base less heavily guarded and ripe for attack.
If your enemy is well bunked with sams, you can always send an orca, JJ, or carry-all in front of your main air attack so that they draw all the fire from his/her sams.