I would like to allow external internet access when connected to the WLAN but block people from accessing the LAN. My current set up is the modem then a firewall then a switch and then the Linksys WRT54GL wireless router. I've tried blocking all ports except for 80 and 443 but that doesn't work. Any ideas?
The WRT is a consumer router built to make home networking for people easier. There is no way to do what you want to do with a WRT. For really secure client separation you'll need a proper wireless access point which supports at least VLANs, and a managed VLAN switch. Then you are free to control access of clients in your network.
Any access restrictions you can set up in the WRT are only for the WAN-LAN connection (where the router sits). The LAN on the ethernet level is a simply unprotected ethernet LAN. There router is not doing much on that side.