How do I set the source address on a multi-homed host?

[Daniela]

I have two NICs (one inside and one outside interface) with NAT activated. The 
problem is that every time I establish a connection with a machine on my LAN, 
it uses the address of the outside interface as the source of the packets, 
which creates problems with my firewall. How do I tell my machine to use the 
other address whenever I connect to a local machine?

Daniela