The JVM is best at freeing short-lived objects. Try not to allocate objects you don't need. But you can't optimize the memory usage until you understand your workload, the object lifetime, and the object sizes. A profiler can tell you this.
Finally, the #1 thing you must avoid doing: never use Finalizers. Finalizers interfere with garbage collection, since the object can't be just freed but must be queued for finalization, which may or may not occur. It's best to never use finalizers.
As for the memory usage you're seeing in Eclipse, it's not necessarily relevant. The GC will do its job based on how much free memory there is. If you have lots of free memory you might not see a single GC before the app is shut down. If you find your app running out of memory then only a real profiler can tell you where the leaks or inefficiencies are.
