Open file /etc/postgresql/9.2/main/postgresql.conf and edit following parameters:
shared_buffers = 512MB temp_buffers = 16MB work_mem = 128MB maintenance_work_mem = 256MB max_stack_depth = 6MB effective_cache_size = 512MB
You have to also configure shmax and shmall kernel parameters, open file /etc/sysctl.conf and add:
kernel.shmmax = 4294967296 kernel.shmall = 4194304
And reload kernel settings by execute:
$ sysctl -p
Restart your PostgreSQL process and enjoy better performance.
Here you can read more about resource consumption in PostgreSQL.