You have a tough task ahead of you. My suggestion is to plan everything on paper. You need to subnet the flat network, create a VLAN for each of the subnets and then do inter-VLAN routing. Apart from that, you need to have a VLAN for common services like apps servers, DHCP, DNS, etc. which need to be reachable by all VLANs. One VLAN should be kept separate for management/administration purposes. Also, keep in mind scalability requirements for the future.
This was first published in September 2006