Go into the router settings and look for DHCP (should be under the networking section if memory serves). You will need the PlayStation 3's MAC Address, you can find this by going to Settings, then System Settings, then System Information on the PS3.
Find the DHCP Client List and look for a button which says to Add a Client. Make the address whatever you like in the range the router is allowed (192.168.0.150 as an example) and type in the MAC Address into the blanks provided. Check off an enabled box (if one) and hit the save/ apply/ ok button. Once done you can edit that same IP address into the PS3's settings.