Getting mobile.
Your main structures move so use this to your advantage when expanding. 

SCVs 101.
SCVs need to completely build a structure and while doing so cannot gather resources.  Make sure you create enough that you do not feel a dent in your resources when putting up structures.

Repair and repair again.
Repair damaged units and structures.   A SCV can actually repair units so make good use of it.

Get mobile.
Your barracks is mobile.  Move it closer to the front and your troops will arrive that much earlier.

It's a siege.
Bunkers backed by siege tanks in siege mode make a formidable front line offense.   Once you have destroyed the buildings within range just drop down 2 more bunkers, move forward and do it all again.

The wrath of the wraith.
Many times defenders will send a hoard of ground troops out to fend off your offensive squads.  Usually there are no 'detectors' with this group so cloaked wraiths can dismember the enemy at will.

A science vessel for any occasion.
Irradiate and EMP shockwave will slow down and severely hurt any defending units.   Once you have softened them up just pick them off with your offensive troops.

I'm a scout, I'm a marine, I'm a scout.
Marines make great scouts for finding out when your enemy is expanding.  Place a few of these cheap units in the empty expansion slots and wait for the fireworks

Can you say bunker? I knew you could.
The terrans can effectively take 4 marines and give them each 350+ hitpoints.  How can they do that you may ask?   Put them in a bunker.   A bunker will hold 4 infantry units.  I tend to use marines because of their attack range but firebats will do as well.  Remember that firebats cannot attack air units. Do the math, 4 marines attacking at a minimum of 6 strength each in a 350 HP bunker.   Ouch.  

Place bunkers at chokepoints and at entrances to your base.  They work best when used in groups as an enemy must take out one bunker while multiple bunkers are shooting back at them.

Terrets Syndrome it ain't but it will make your enemy swear.
Missile turrets have a very fast rate of fire and are relatively cheap to drop down.   They can also detect cloaked or concealed units.  Use them in strategic locations to keep the undesirables at bay.

Steven Seagall eat your heart out.
The term under siege will mean nothing until your enemy is pummeled by siege tanks.   Placing siege tanks behind bunkers (in siege mode) will devastate most ground units before they can even try to take out the bunkers.

War is a science vessel.
A science vessel or four can cause the enemy no end of grief.   Irradiate will annoy any incoming Zerg forces and EMP will remove the shields from the Protoss attackers.  After that you just mop up the remaining resistance.

Caught in your web.
Most maps have a limited number of base entrances.  Since you will undoubtedly have bunkers and siege tanks waiting for the incoming ground units you might as well let them trip on a few spider mines as well.    Lay spider mines a short distance in front of your bunkers to soften up the attackers.