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

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() ?