This is a difficult problem in general. It is made even more difficult by the fact that many things which appear to be leaks (because you can see memory usage growing) are in fact not leaks at all–Panda maintains a number of caches internally to improve performance, and it is normal for memory usage to grow as these caches fill up. However, eventually the caches will reach their limit or be emptied, and memory usage will stop growing.
Many people ask about similar questions from time to time. See, for example, this thread.
David