Originally posted by Christian Haselgrove:
If you want to relicense the code you have to figure out everyone who has copyrights on parts of the code to relicense and get their permission. For large project it's almost impossible (Linux is an example where Linus Torvalds explained why moving from GPLv2 to GPLv3 would require asking thousands of people for permission, including the dead developers). For small projects you might be the sole author and thus have the copyright on all the code and be able to do it. Some projects like MySQL and most of the GNU tools ask for copyright reasingment from developers. This is an option but many developers are against this completely. You could check the document then the FSF ask people to sign for submiting code to GNU projects and also speak to a lawyer if you want to go that route. 

The GPL has a nice little explanation on what to put where : http://www.gnu.org/licenses/gpl.html (look at the bottom of the page) it's a similar process for BSD code. You should put a copyright statment and license statment in all your files. Alternatively some project put it in a LICENSE file at the top of their source code tree. 

To be compatible means you can take code under two different licenses and use them together in a project. For example if you create a new project that under the GPL and want to use BSD code for some stuff that would be fine. The opposite won't work (BSD project using GPL code) since the GPL requires all derivative code to also be GPL. Librairies usually will use the LGPL to allow the projects using it to choose another license. The LGPL isn't as broad on what constitute derivative work.