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Terran Advanced
If you survive the early game stage, your main goal is to upgrade as fast as possible and slow down your enemy as much as you can. At this stage you must watch over your supply - you'll need to have 10 unused supplies at all times. Here are some of the mid game tactics that can be a good way for a Terran player to slow down his opponent. Nevertheless, if you see that your opponent has too weak defence, which can be penetrated, don't hesitate to attack. 

Vulture Drop
Vultures aren't much of a powerful unit, but you probably never noticed that they do concussion damage. Than means they damage small units the best - the whole of 20 damage points. By dropping 4 vultures among your enemy's workers you will be able to kill at least some of them before being noticed. When you do notice units moving towards you, lay out some spider mines to halt them for a while, and continue shooting at workers. If all the workers retreat, lay a few mines and come back in half a minute (spider mines are not triggered by workers, however, they will kill it if they explode next to them). This is like very good way to slow down your enemy, sometimes without any losses.

Siege Tanks Dropping
Siege tanks are a powerful but not so strong unit. They usage is best on the maps with higher grounds looking down on shorter passages (look at The Lost Temple and you'll see what I mean). You can practically kill an enemy attack before it comes. On the maps that have expansions near higher grounds it's very good to use a tank drop. Load one of your dropships with a siege tank, 3 marines and one medic (scv if playing regular starcraft) and drop it on the higher ground over expansion. Repeat the process once again to have 2 tanks and then set them to siege mode. This way you'll kill most of his workers and units before noticing. If you see a transporter (dropship, overlord, shuttle) comming, stim your marines and shoot it before it unloads it's cargo.

Using the Irradiate
Using irradiate can sometimes win you a game. It is my recommendation to upgrade irradiate as soon as possible. Against Zerg, with cost of only 75 energy, you can kill an overlord, meaning that if your science vessel has 225 energy, you can kill 3 overlords without any losses whatsoever. By building 2 or 3 of these ships, you can kill all of your enemy's overlords in which case he won't be able to produce any more units. Casting a irradiate on workers can kill at least one of them but casting like 3 irradiates on 3 different workers can sometimes kill off 10 and more workers.

"Newbie" Wraith Flanking
Well, this one is not so smart strat, but it works. Make about 12 wraiths and split them in two groups the 1st consisted of 3 and the 2nd of 9 wraiths. Use your 1st group to attack the your enemy's main base (not the workers). Position your 2nd group next to your opponent's main resource base. When the anti air units come to the 1st group, cloak them. Meanwhile, move in the 2nd to kill all his workers in his resource base. After that retreat the 2nd, without losses, and if the 1st one is still alive, kill some workers or things like that.