(BTW, many are about to tell you that the device is old and that the firmware should not be ran in a production environment due to the many security vulnerabilities in the software - discovered over the years.)
Example - flash to 10.03.1:
wget http://downloads.openwrt.org/backfire/10.03.1/brcm47xx/openwrt-brcm47xx-squashfs.trx # The file must be the trx file.
mtd write /tmp/openwrt-brcm47xx-squashfs.trx linux && reboot
The best thing to do with a Linksys WRT54GL is to try to find a technology museum to donate it to.
The second-best thing is to recycle it.
As the firmware on the device has been severely insecure for a years, and the OEM firmware was never secure, downgrading only throws fuel on the fire.
Suitable replacements (at least 16 MB flash, at least 128 MB RAM, support of current wireless standards) can be obtained starting around US$20, new.
Enthusiastic experts may be able to get a tiny bit of utility out of the long-obsolete, completely unsupportable in a secure manner WRT54g-series devices, but not as a robust, secure router. The availability of inexpensive replacements that outperform it in every way makes such use little more than an academic exercise.