Most memory leaks are caused by incompatible/ flakey effects, damaged CoDecs or specifics in the footage and effects used. Since you did not provide any info and also haven't provided any system info, nobody can really help you...
Sorry, the system info is the following:
i7-2600, 3.80Ghz; NVidia GTX 560
16GB RAM, Windows 7, 64 bit.
So the system is rather fast and I don't use multiprocessor rendering. I also don't use disk cache at all. The effects I use are not common at all, so it would not be useful to talk about them.
But the main question was not about memory leaks, I just wanted to learn the way to force After Effects to purge cached memory.
Thanks, this update fixed my problem.