EPT*This guide is for lan play only. Many of the stuff here do not apply for b.net games because of lag.
If you watch replays religiously you may have noticed a recent trend of "sick mutaing" by many gosu korean zergs. Many are so confident in their harass that they even open 2 hatch muta and skip lurker to try to kill terran through harass alone. Some examples are
http://sc.gosugamers.net/replays/11268
http://www.yaoyuan.com/show.php?SID=55277
http://www.yaoyuan.com/show.php?SID=55236
http://www.yaoyuan.com/show.php?SID=55189
http://www.yaoyuan.com/show.php?SID=55100
http://sc.gosugamers.net/replays/11047
As you can see with 1 rax fast expo, if terran does not defend perfectly for the first minutes vs the initial wave of mutalisks they can die without ever getting to mid game. I have personally been murdered countless times by pure mutalisks even while holding big advantages before the mutas are out, a particularly gruesome example is here http://www.teamliquid.net/forum/viewmessage.php?topic_id=63603 , truly one of the most embarassing games ever.
Last night after experiementing with many things including going valkyries to 6 rax before cc, I discovered the best way to defend vs sick mutaing is to just simply, micro well. You have to control your marines in specific ways to avoid dying needlessly. A couple of thing I realized are:
1. Attack only one muta at a time. If your opponent is truly a patron of "sick mutaing", he will turn his mutas away instantly after the spikes come out, giving your marines virtually no time to attack the muta clump. But given that 8 mutas and 10 mutas differ by a lot, 10 mutas can kill two marines almost always with one good hold, 8 rarely can, you should always try to deal the most damage in the shortest time span possible, which would mean to attack one muta at a time.
2. Try not to stay still when mutalisks are near your marines. When zerg players practice sick mutaing, most use only two methods, attack-move + left-click and hold. For both the player has to first right click within range of the object the player wants to attack and then perform the command he wants to do. The absolute key to sick mutaing is to turn away as soon as the command takes effect, which means when the spikes come out, and because of this, zerg players will only press the attacking command once and then right-click away immediately after; which means if the attacking command does not take effect, that is if the mutas do not attack, they have to turn away anyways because the right-click command has already taken effect. So in turn, the procedure for a muta raid is to right-click until mutas are within range of marines, hold or attack-move once, then right click away.
From the terran player's perspective, the logical reaction would be to make the mutalisks miss as often as possible that one attacking command the zerg player uses. To do that, terran has to move their marines away from the mutalisks' range when you're not in good formation and when your marines are too few to deal any damage. The most important aspect of this is that stimmed marines move faster than mutalisks. So by stimming and moving away suddenly in the direction parallel to how the mutas are coming at you, you can most of the time make the muta clump miss because when the zerg uses the attack command the marines are in range but when they actually attack your marines will be out of their range.
3. Attack in good formation. A good formation is when a lot of marines stand perpendicularly to mutalisks
Good formation:
xxx
xxx
xxx _________________ x <- mutas
xxx
xxx <- marines
When you're in this situation you should retreat when the mutas are coming at you and then move towards mutas when they are turning away and then aim at one muta. All while stimmed. Because of this formation, most of your marines will be in range of the muta cluster therefore able to deal a lot of damage in a short time, and because stimmed marines are faster than mutalisks you will be able to catch up to the mutas when they're turning away to attack, and then quickly turn back when the mutas attack you. It is much easier said than done. But this should be your basic philosophy to dealing with muta raids, retreat when being attacked, and attack when mutas retreat.
Bad formation:
xxxxxxx <- marines ______________ x <- mutas
In this formation, the marines that stick out in front are asking to be killed. They are far away from medics and when you attack, and only a few will be in range of the mutalisks when you attack. With one good hold, mutas can kill most of the ones that are sticking out, and the ones that don't die will have to catch up to attack, and then they will be killed too.
4. Don't stim too much. Don't stim when there are no medics nearby, don't stim twice in a short period. Obviously this is because clumped mutas will deal way more damage to stimmed marines and with splash, many will die.
5. Do not chase when your marines are stimmed and in bad formation. This goes along with 2 and 3. Especially when you're attack moving and not aiming for one muta and when you don't have a lot of marines.
6. Have lots of medics available, an extra medic or two will really cancel out the splash damage, saving your precious marines from otherwise certain deaths.
These are all the things I can think of right now. I can't really say much about turret placement since I'm still kinda bad at it. But the general idea is to place turrets to set up a perimeter around your base so mutas don't go inside that boundary instead of only placing turrets near your scvs.
I hope these tips give terran users a better idea of how mutalisks attack and what to look for in attacking. And also I hope zerg users understand more about sick mutaing and what to look for in harassing. The only example in progaming that I know of a terran doing well vs harass is the proleague game where Jaedong lost his winning streak vs Sheis
You can see Sheis controlling his marines well so Jaedong's 2 hatch muta did virtually no damage. This kind of marine control requires a lot of attention and multi-tasking. But same for the zerg with good harass. I hope you found this guide helpful, thanks for reading.
, but I still play and can go on hamachi/ggc, etc, have you started playing again?
. But it requires way too much hand speed, so for now I think to use what I always have used and what skyglow1 pointed - when I see mutas coming in for a shot - stim and engage with right click so I can take as many mutas as I can in the first fight and reduce their damage as much as I can in a longer harass when the mutas start to fly over my base trying to get scvs 
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