It is not common, nor is it expected, to tip in Korea.
If you wish to give a tip to your masseuse, hairdresser, or to someone who has gone above and beyond to help you, you can leave a 10-15% tip and it will be greatly appreciated. However, tipping is generally frowned upon and the majority of people in restaurants or taxis would be offended if you offered them a tip. The one exception is when you eat at a more up-scale restaurant. Don’t be surprised if there is a V.A.T added to your bill. This is a kind of gratuity that goes to the staff (wait staff and chefs), and, in this case, is very common.
If you lose something important, such as a cellphone, wallet, or your laptop, Koreans are generally very honest and will not steal from you. Indeed, they usually go to great lengths to find the owner of said item. It is customary, then, to give a small reward for their honesty and kindness, usually 20 or 30,000 won, depending on the value of the found item.
Koreans are always trying to compete with competitors for your business. Therefore, many establishments will give you “service”. Generally speaking, you may often receive complimentary items with regular purchases. It’s great a great aspect of Korea!