How should I implement my chosen software license?

Question

I have always kind of wondered, but never took my time to actually ask these questions regarding licensing. Most of these questions are presumably pretty basic, but I have no idea what I'm dealing with here.

The thing that baffles me the most is how exactly you "implement" the license for your OSS. Is just saying in the post you're publishing the code on, "by the way, this is licensed under GPL" enough? Or should something more delicate take place, like registering the software somewhere? Also,

• If I just say I use a particular license, is that enough?
• Do I add a copy of the license to the distributed code?
• Am I required to add a header on every file detailing the licensing information? Do I manage this with my version control system (e.g. Subversion)?
• Is the above not enough?
• What are the most commonly used software licenses?
• Are there any gotchas/licenses that are better left alone?