How to protect yourself against drops

It is too often in my games (in TvT) I detect that my opponent after getting some good defended wall building StarPort and preparing to do a drop. I understand, that 1st thing to do is to look on the mini-map. But often also in this case you need to move your army to defend... it takes time...

Question 1: How can I prepare to defend from that in the best way if I know that it is planned and will be done soon?

Question 2: How can I be more or less defended against unexpected drop (for example, I could have 1-2 siege tanks in siege mode on my base/natural)?

P.S. Here I am interested in play any race against any, but if you have good specific examples, it will be good too.

Thanks.

Don't ask yourself "how" to defend but ask yourself "why" that he plans a medivac drop.

Impressive medivac play is shown in Day[9] Daily #248 - Dealing with Duckload (14:17 - 27:00),
let's see at what we have to note and can learn from watching this video:

• Long Rush Distance, so Protoss techs to Colossi to counter a Bio Ball.
• Terran knows Protoss is building up a strong army, so he must prevent the attack.
• Constant harassment with Medivac Drops allows Terran to expand back home.
• Protoss has to split his army to cover all sides and can only harass with a Warp Prism.

Ugh... What went wrong at Protoss' side?

• Colossi are slow to handle Medivac drops, they would be more useful later in the game.
• Terran knows nothing is going to counter his Medivacs, as Protoss has made a Robotics Bay.
• Because Protoss plans a defensive build, Terran knows that there won't be early harassment.
• Colossi are slow and 2 Gateways don't allow for much units, so Terran splits his army.

Ah... What could Protoss have done instead?

• Air: Early Phoenix Harassment (and to hunt the Medivacs) and/or building enough Void Rays.
• Templar: Stalkers can Blink, HTs can handle the Bio Ball and DTs are handy for harassment.

As for how, you'll find the answer in the Help in-game or at Liquipedia II, there isn't much more to say:

How can I prepare to defend from that in the best way if I know that it is planned?

If you learned why you'll have a better composition to handle the issue.

We could list any combination, but inspecting what went wrong will learn you how to do that yourself...

How can I be more or less defended against unexpected drop?

Scouting and map control are key. Don't mindlessly attack, but know what your opponent is doing...

Note: If you plan to use Tanks, have something to counter air or else he'll simple drop on your tank.

Playing as Terran, Sensor Towers + Xel'Naga Towers are not an unreasonable method of obtaining advanced warning of incoming drops. Outside of complete map control or having a surround on the opponent, this should at least give you more warning than hearing "Your SCVs are under attack".

In terms of defense at the mineral line, missle turrets + siege tanks is one route. You could also throw down a couple of bunkers and fill them with a marine/marauder mix for for a defense against a great spectrum of dropped units. This is dependent on the size of the drop, however, so it's not safe to assume you can leave a set force always on your mineral line and expect to be completely safe from drops. If your opponent goes all-in and drops his entire army on your mineral line, you're either forced to pull back or go for the base-race victory.

To avoid this, you want as much scouting and map control as possible. Don't be afraid to use Sensor Towers, especially if your opponent has already scouted you. You're not telling them anything new aside from "I see what you're trying to do there". It's an easy way to get more map visibility without spreading yourself too thin.