Too many open files! how do I fix on linux?

Hi again, I'm getting stuff like

ERROR:  cannot write block 1056 of tradehistory [testdb] blind: Too many
open files in system

and

postmaster: StreamConnection: accept: Too many open files in system

I understand that I need to up the max number of open files in the linux
kernel but I'd like to know what's a good number to set it to for a high
traffic postgres server and what's the best way to set it in linux so
it's permanent?

(These are during some performance tests I'm running with about 40
simultaneous clients)

Thanks,

Alex.