Force JVM to garbage collect externally?

I'm running PS3 Media Server on Arch Linux, and it's using quite a bit of memory that I'm sure could be collected if the JVM would just try. Is there any way for me to get the process to garbage collect? Maybe I there's a signal I can send to it?

Depending on how it is invoked, maybe the JVM can be told to prefer to garbage collect more often instead of expanding the heap? Isn't there a flag to set the maximum heap size?