In the LuCI web interface, select Administration, then Network / DHCP / Leases. Add your (MAC address, IP address) entries to the Static leases list. Click on Save & Apply. Ask for a new IP address at your computer. (On Debian and Ubuntu, do an ifdown eth0; ifup eth0; on Windows, do an ipconfig /renew /all, or use the Control Panel to bring the network interface down and then up again.)
Should you not get the static IP address you've set up, double check that you have specified the MAC address correctly. If you still don't get it, SSH onto the router, and run the following commands:
root@OpenWrt:~# /etc/init.d/dnsmasq stopOnce this is done, ask for a new IP address at your computer.
root@OpenWrt:~# true >/var/dhcp.leases
root@OpenWrt:~# /etc/init.d/dnsmasq start