Terran Final
When the game is in it's late stage, Terrans become very powerful. Just by using battlecruisers, tanks or even just marines, you'll sometimes be able to completely crush your opponent. I mentioned marines because if you upgrade them to their maximum (also range and stimpack) you will have so powerful units that can crush even zealots in overwhelming numbers. If you have played correctly in the middle game stage, you shouldn't have much trouble here. If you played against someone with the same skills as you, however, it is difficult to say when to attack. Roughly, you two will have very similar unit count, meaning if you do attack you will be overpowered until your troops reach the enemy. In this case you will need to rally on some different attacking strategies such as the ones I will mention here.

General Flanking
You all probably know what is considered by flanking. However, a common mistake is splitting your army in two parts and attacking different places. The most important thing in flanking is unit the choice - you will have to use 4-5 stronger units to attack an expansion while with your main force you are ready to crush your enemy. I usually use 3 tanks with 2 cloaked wraiths, although not so cheap, this is a powerful flanking group. When 3 tanks starts shooting from the distance, most of the players will drag their units to them in which case I unsiege them and flee to different sides. The sam moment I begin to attack with cloaked wraiths, forcing him to bring an detector and some more units. When my wraiths are about to be destroyed I commence my main attack on his main base. In this case, I will severely cripple him until his units show up. Following is retreat (if my forces tend to be overwhelmed) and while his forces are following mine I usually do a drop consisted of 12 marines and 4 medics with whom I destroy his crucial buildings (cybernetics bay/pylons/forge, spawning pool/hydralisk den/spire, academy/command centres). In this moment, he would be just in front of my base and with some micromanagment with siege tanks/bunkers/yamato and with units build in the meantime I would usually beat my opponent.

Using the DropShip
Drop ship can be used in many different ways as can be any transporter unit. If you plan an attack you can use your dropships for flanking - bring 2-3 dropships within your enemy's sight range and do a scanner sweep as you would do if you were preparing your main attack. If and/or when you see his forces coming, do your main attack. This will usually give you a distinct advantage over your enemy and lead you to victory. Other use of the dropships would be the real drops :). A single drop would probably be easily killed but if you'd do a drop on every expansion your enemy has, it would be much better, especially if done in completely same time. For protection, you can bring one wraith with you what works pretty well combined with scanner sweeps.

Marines Push
This isn't really a late game tactic but it usually tends to drag into it. After you build two barracks in the beginning, expand, build refinery, academy and comsat. Then build two more barracks and expand. Build two more barracks and expand (you see the pattern now), and so on. The thing about marine push is that you constantly keep cranking marines in all of your barracks and attack whenever you have 12 of them. This will usually force your opponent to go all out defence and probably, he'll be afraid to expand. When he does get the force necessary to expand, you'll already have 3-4 expansions and 6-8 barracks. Now just pile up 48 marines and 2-3 tanks and the game is yours.

Terran Spells
Terrans have very useful spells. I mention something about irradiate before. Another very useful spell is the lockdown. Terran worst enemies - carriers, arbiters, reavers, battlecruisers, tanks, and other mechanical units can be easily stopped and killed. Especially usefull if your enemy is going for a drop - with 2-3 ghosts you can split his force to 2, or even completely stop him. Medic has a few useful spells, in the first place healing. Blinding, sometimes good, has very limited use. You can use it to blind your opponent's detectors, but on the other hand, terrans don't have much powerful cloaked units. Perhaps the best use of it is on reaver drop - you can blind the shuttle and then both reavers, what should slow it quite a bit. EMP shockwave is sometimes an useful spell, especially against protoss. The best it's use is if you see a packed group of corsairs approaching your position you can both drain all of their energy and shields as well. Also for packed groups of ground forces, EMP shockwave works pretty good in combination with irradiate. Protection field created by Science Vessel is one nice but not so good spell, it gives the affected unit a 250 HP for an period of time. This can be useful for scouting, then againt there's comsat for this. If used in attacks, these added 250 HP usually won't even be noticed.