I probably start up a JVM a thousand times a day. Test runs, benchmark runs, bug confirmation, API exploration, or running actual apps. And in many of these runs, I use various JVM switches to tweak performance or investigate runtime metrics. Here's a short list of my favorite JVM switches (note these are Hotspot/OpenJDK/SunJDK switches, and may or may not work on yours. Apple JVM is basically the same, so these work).
via blog.headius.com
If you write any serious Java at all, I highly recommend that you click the link above and read his post. It is full of many amazing gems that I can see being really useful for optimizing and debugging Java applications.