On CentOS and RHEL Linux (with kernels >= 2.6.32) you can modify resource-limits (ulimit) run-time. This can be done using the /proc/<pid>/limits functionality. On older kernels this file is read-only and can be used to inspect the limits that are in effect on the process. On newer systems you can modify the limits with echo:
echo -n "Max open files=soft:hard" > /proc/pid/limits
On older systems you will have to modify limits before starting a process.
(See also the post on serverfault)
If you are not running CentOS/RHEL, you can use the ‘prlimit’ command, which does the same thing, but doesn’t rely on a patch that’s no longer present in current kernels.