[CentOS] IP forwarding and OpenVPN

I have a CentOS 4 server that acts as a gateway for a small LAN.   The
lower half of the 192.168.144 address space is the LAN on eth1, the
upper half is the WAN on eth0, and the default route is to
192.168.144.254 which is my DSL router; this has been working fine for
years.

However, it's recently become convenient to connect the server to a
VPN from time to time, for which I've set up OpenVPN.  This works for
connections originating from the server itself, but breaks for
machines on the LAN when accessing IPs in the ranges that are routed
to the VPN.  Connections to IPs not in the private network still work
as before.

Can anyone advise what I may need to change to configure the server to
forward packets to the VPN?  Pointers to documentation are welcome.
Thanks.

/etc/sysctl.conf has:
net.ipv4.ip_forward = 1
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0