Out of Memory

I am running an application i wrote which crawls a very large file system and keeps information about files in a data struncture, a hash table. The program runs out of memory and i get an exception. Is there a way to save memory? I used to write smaller hash tables and save them in many files on the disk which in fact saved memory and made the program run more smoothly. Is there any better way to save memory that does not have so much overhead? What is the solution generally followed in these circumstances?

I dont want to give appliacation more memory than the operating system allows, i need a good design pattern instead.

Thanks