GC.Collect() not cleaning memory, how to find out what references to lots of memory still exist?

GC.Collect() not cleaning memory, how to find out what references to lots of
memory still exist?

When all my processign is done i set everything to null and then:
GC.Collect();
and then
GC.WaitForPendingFinalizers();
but it still shows that my process takes 400 MB of memory. Is there any easy
way to see what references that I forgot to set to null so that the memory
cleas up on GC.Collect() ?