When it comes to performance tuning, how do I get the most out of GlassFish?

The GlassFish / Sun Java System Application Server documentation set includes a complete chapter about performance tuning in the Performance Tuning Guide.

But if it's a quick tuning cheatsheet you need, Scott's blog entry A GlassFish Tuning Primer is just what you're looking for! As described on The Aquarium, it provides a good one-page summary and will whet your appetite for the detailed information in the docs.

You can also use the Performance Tuner part of the GlassFish Enterprise manager. In particular, see this documentation page.
Note the Enterprise manager is available to customers only.