Help me stop postgres from crashing.

Hi,

I am a web developer, I've been using postgesql for a few years but
administratively I am a novice.

A particular web application I am working has a staging version
running one a vps, and a production version running on another vps.
They both get about the same usage, but the production version keeps
crashing and has to be re-started daily for the last couple days. The
log file at the time of crash looks like this:

LOG: could not accept new connection: Cannot allocate memory
LOG: select() failed in postmaster: Cannot allocate memory
FATAL: semctl(2457615, 0, SETVAL, 0) failed: Invalid argument
LOG: logger shutting down
LOG: database system was interrupted at 2010-04-24 09:33:39 PDT

It ran out of memory.

I am looking for a way to track down what is actually causing the
memory shortage and how to prevent it or increase the memory
available.

The vps in question is a media temple DV running CentOS and postgres
8.1.18

Thanks.