Great breakdown on a macro-level view, YMG. I'd add a few things I observed from my time working in Korea as many of my friends were in the industry.
1. Be very judicious in picking your place of employment. Tutoring institutions are wildly across the board in quality and the way they treat their employees. Several people I know worked for really high end tutoring joints in ritzy areas. Pay was commensurately better (45-50k year starting), hours were decent, and classroom sizes manageable. However, there are other smaller places where they crammed 50 kids into classrooms, gave shitty 200 sq ft. "apartments" as housing and made teachers do "split shifts" where they would work from 8am-1pm and then from 6pm-11pm which counted as one day. Lots of scandalous stories about tutoring centers also pulling employment contracts in the 11th month so as not to have to buy plane tickets for teachers back home. Limit yourself to applying to large, well established schools like Pagoda, Wall Street and Chung Dahm Institute and this should limit most of the scammers out there. Of course, these places are harder to get into, so make a good impression.
2. Seoul is a mix of cheap and expensive. If you want to mimic your life in the US or Western Europe, your teacher's salary isn't going to cut it... Bottles of single malt whisky are $500 US, western food is expensive, imported goods have huge taxes and duties which bump up the price 30-100%... but if you're cool with eating a lot of local food, drinking the insanely cheap local liquor and beer, you can definitely squirrel away at least 50% of your income. This is largely due to getting free housing from the tutoring institute and not having a car.
3. I'm going to have to disagree with YMG's take on getting an internship while you're teaching. Having seen many friends go through the teaching route, you're going to be working a minimum of 50 hours a week in addition to some take home work. There really isn't a very well established intern culture in a lot of companies, at least when I worked for a multinational in Korea, and even if they did have an internship, it'd likely be for a local Korean speaker and they'd invariably demand 40-50 hours a week. Having said that, your best bet to make extra scratch is to try and get private tutoring gigs on the down low so as not to violate your contract with your 9-5 job. More on that later...
4. Be careful that you don't fall into the wrong crowd. It seems like English teaching is easy enough for anybody to get into, so you tend to get the dregs of Europe, South Africa, Oceania and the US come to Korea to teach. Lots of these idiots have no ambition and just bring you down with their constant drunkenness and coasting through life.
5. White is right... most of the time. By that, I mean that you are going to have a huge advantage getting hired if you are Caucasian. A distant second is Korean American, even if you speak perfect American English without a Korean accent, they're going to think you somehow have flawed English. Other Asians, Blacks, Latinos, Middle Easterners and the disparate groups I've left off are left with an uphill climb in not looking white (again even if they speak perfect English) and not being Korean. This is why you see so many white Scottish teachers there, although their brogue is extremely hard to decipher, even for a native English speaker. Side note: the only time being a Korean/American teacher is advantageous to being white is picking up private tutoring jobs that pay north of $50/hr. Parents would prefer to have some sort of linguistic dialogue with their teachers after they instruct their children. Plus, a lot of them would probably feel uncomfortable letting a barbarian foreigner into their homes, no matter how well mannered. Yes, Koreans are racist as fuck...
6. Look out for arbitrage opportunities. My Korean American friend that lives there survives by banking on new trends. In the early 2000's snowboarding got huge in Seoul, with people paying upwards of $1200-$1500 for entry level boards. My buddy seeing this booked a $800 ticket to LA, attended the Ski-dazzle show at the convention center and bought $5000 worth of Burton gear there with huge volume discounts on already low prices. Got it all through customs duty free by saying that he was a professional snowboarder and sold the gear within a week for nearly 10 times the money. Clothing, car parts and electronics (going both ways) are good things to keep your eye on, although some may be more scaleable than other things.
Having said that, I worked 55 hours a week at my corporate job, but made an extra $1500 - $2000 a month teaching kids and businessmen English on the side, cash money. Lots of these sessions involved meeting at cafes for conversational practice where the students would always pay. I still have a few friends who annually clear $150k cash just doing private lessons, and although there's no job future in it, and they're in the minority, it's a definite possibility.
1. Be very judicious in picking your place of employment. Tutoring institutions are wildly across the board in quality and the way they treat their employees. Several people I know worked for really high end tutoring joints in ritzy areas. Pay was commensurately better (45-50k year starting), hours were decent, and classroom sizes manageable. However, there are other smaller places where they crammed 50 kids into classrooms, gave shitty 200 sq ft. "apartments" as housing and made teachers do "split shifts" where they would work from 8am-1pm and then from 6pm-11pm which counted as one day. Lots of scandalous stories about tutoring centers also pulling employment contracts in the 11th month so as not to have to buy plane tickets for teachers back home. Limit yourself to applying to large, well established schools like Pagoda, Wall Street and Chung Dahm Institute and this should limit most of the scammers out there. Of course, these places are harder to get into, so make a good impression.
2. Seoul is a mix of cheap and expensive. If you want to mimic your life in the US or Western Europe, your teacher's salary isn't going to cut it... Bottles of single malt whisky are $500 US, western food is expensive, imported goods have huge taxes and duties which bump up the price 30-100%... but if you're cool with eating a lot of local food, drinking the insanely cheap local liquor and beer, you can definitely squirrel away at least 50% of your income. This is largely due to getting free housing from the tutoring institute and not having a car.
3. I'm going to have to disagree with YMG's take on getting an internship while you're teaching. Having seen many friends go through the teaching route, you're going to be working a minimum of 50 hours a week in addition to some take home work. There really isn't a very well established intern culture in a lot of companies, at least when I worked for a multinational in Korea, and even if they did have an internship, it'd likely be for a local Korean speaker and they'd invariably demand 40-50 hours a week. Having said that, your best bet to make extra scratch is to try and get private tutoring gigs on the down low so as not to violate your contract with your 9-5 job. More on that later...
4. Be careful that you don't fall into the wrong crowd. It seems like English teaching is easy enough for anybody to get into, so you tend to get the dregs of Europe, South Africa, Oceania and the US come to Korea to teach. Lots of these idiots have no ambition and just bring you down with their constant drunkenness and coasting through life.
5. White is right... most of the time. By that, I mean that you are going to have a huge advantage getting hired if you are Caucasian. A distant second is Korean American, even if you speak perfect American English without a Korean accent, they're going to think you somehow have flawed English. Other Asians, Blacks, Latinos, Middle Easterners and the disparate groups I've left off are left with an uphill climb in not looking white (again even if they speak perfect English) and not being Korean. This is why you see so many white Scottish teachers there, although their brogue is extremely hard to decipher, even for a native English speaker. Side note: the only time being a Korean/American teacher is advantageous to being white is picking up private tutoring jobs that pay north of $50/hr. Parents would prefer to have some sort of linguistic dialogue with their teachers after they instruct their children. Plus, a lot of them would probably feel uncomfortable letting a barbarian foreigner into their homes, no matter how well mannered. Yes, Koreans are racist as fuck...
6. Look out for arbitrage opportunities. My Korean American friend that lives there survives by banking on new trends. In the early 2000's snowboarding got huge in Seoul, with people paying upwards of $1200-$1500 for entry level boards. My buddy seeing this booked a $800 ticket to LA, attended the Ski-dazzle show at the convention center and bought $5000 worth of Burton gear there with huge volume discounts on already low prices. Got it all through customs duty free by saying that he was a professional snowboarder and sold the gear within a week for nearly 10 times the money. Clothing, car parts and electronics (going both ways) are good things to keep your eye on, although some may be more scaleable than other things.
Having said that, I worked 55 hours a week at my corporate job, but made an extra $1500 - $2000 a month teaching kids and businessmen English on the side, cash money. Lots of these sessions involved meeting at cafes for conversational practice where the students would always pay. I still have a few friends who annually clear $150k cash just doing private lessons, and although there's no job future in it, and they're in the minority, it's a definite possibility.