The ASA/PIX does not support PBR to date. I've been told its on the roadmap.
As a work around, you could run multiple contexts, where contextA inside = 192.168.1.0/24 and contextB inside = 192.168.2.0/24
And also allocate the appropriate Internet interfaces to each context (with the default gateway pointing to the respective ISPs).
This link will get you started:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00808d2b63.shtml
PLEASE NOTE: Dynamic Routing and VPNs are not supported in Multiple Context mode.
Another alternate, if the WAN links are terminated on a router (and not the firewall), you could use that router to do the PBR.
Regards
Farrukh