How to increase shared memory buffers?

Hi All,

I have been trying to increase the shared memory buffers used by
Postgres by using the -B option to postmaster, but postmaster fails to
start up for any value greater than 64 (which is the default in any
case) saying "couldn't get shared memory - permission denied".
So, (I know this should be asked on a Linux group, but...), does anybody

know how can I get my Linux system to allow Postgres more shared memory?

Thanks,
Anand.