I'm not sure about the router that you have, but on my Linksys routers, if you go into the DHCP settings area, you may find a way to assign a static IP address to a device based on the MAC address of that device. Each time the device requests an address, the router will provide only that address, and will not assign that address to any other devices.
Another way to handle this, if you can't find those settings, is to assign a static IP to your printer using the configuration settings of the printer itself. Make sure you pick an address outside of the range of addresses that your routers DHCP server provides. You can determine that range from the routers DHCP settings panel. The advantage of this is that your printer will never drop its address, and will never have to renew its lease, since there isn't one. And, you always know what IP address to point your browser to when you want to change configuration settings on the printer.
As I said, there are a number of ways to do it, take your pick.