I never really write in to anywhere or anything like that, but in reality I am more likely to tell stories about good service than bad.
For example, I recently wrote a glowing a review of a little diner in Syracuse because the service was just so good. I tip generously for very good meal service and tell my friends about it. I don't really complain about bad service except to say kyahbean.
You often get free stuff, discounts, or refunds when you complain about stuff
Didn't you complain about hot pockets once or something?
I've never sent in emails, but when I do get exceptional help or service I tell that person I really am grateful and appreciate it. If I get bad service, I usually just don't say anything and get out as fast as possible.
There have been a couple times when I got great help and the manager/boss was around, so I made sure he/she either heard me complimenting the person or I would go over and make a comment directly to him/her.
I prefer to thank the person who helped me in that moment, but I like sending in emails or letters afterwards too because getting praise from a manager is likely to be a positive experience in their day. I like to think that maybe my praise will lift their mood a few days later.