Generally goodbyes is time to part ways or change the routes, a goodbye letter is written when you do not want to keep a relationship or you do not want to continue a job. In both cases the letter should be written in different style. In first place you are terminating a relationship or a bond that is your personal choice and the letter should be informal. You can write an informal goodbye note according to the relation and kind of problems you have gone through, this is a personal letter. In other case, when you are leaving a job and writing a goodbye letter, that is a formal letter and should be written in a professional way.
Despite of the fact that which type of goodbye letter you are writing, formal or informal, it should not be quick, this way is considered unethical. Some important points should be considered while writing a goodbye letter, first of all consider your audience or your reader. It means to whom you are writing this letter, it will suggest and help you a better way, how to write your letter. You should decide before hand that what you want to write and what should be the mood of your letter. If you are writing a formal goodbye note to your colleagues or coworkers, keep your tone friendly yet professional. If it is for friends or informal and you are not writing a permanent goodbye note, keep the optimistic and light.
You should mention some good points or things as reference, for the time you passed together, at the time of goodbye, its better to mention some good memories or good times you had together. Point out that how worthy was the time with the recipient (s), and also that you had some good time in shape of pleasant memories, do mention your contact details, with a note that you want to stay in touch. Here you can find a sample of goodbye letter for your guidance, with this template that will help you out to create your own goodbye letter.