As of ADT 22, using Eclipse Juno
I don't think any of the above are really the best answers. I also don't think it is recommended to use the external jar function in Eclipse anymore (AFAIK).
Rather, what worked for me is to create a separate empty library project.
Then use Android tools > Add Support Library to get the latest version you need or want.
Then remove the support lib jar from all other projects.
Finally, for every project that requires it, add a reference to your new Library project
(project properties) > Android > (Library box) > Add...
Then all your projects will have a single source to use and update the support library. This also makes javadocs easier to get working.
For info on how to setup the javadocs see:
How to attach javadoc or sources to jars in libs folder?