Before you start this process head over to the network tab on the XMB and choose "Internet Connection Status." From here note your current IP address, your subnet mask, your default router and primary and secondary DNS settings. Write these down, you will need them in the following steps.
Go to the Internet Connection Settings tab on the XMB.
From here most of the settings can simply be passed through with default settings. There are two areas, however, that you have to change. The first is choosing a Custom address setting (instead of Easy).
From here, you will want to choose Manual IP Address Setting:
Remember the settings you wrote down from your status page? You will need them here. You will have to enter in the static IP Address you want to assign to your PS3, the Subnet Mask (which is usually 255.255.255.0), the default router (the IP address of your router, most often 192.168.1.1 or 192.168.100.1), and the Primary and Secondary DNS settings. You will have to input all of these numbers. This is made much easier with a DVD Remote or keyboard.
Please NOTE - You will want to set the last number of your IP address high enough that other wireless or wired items on your network will not be assigned the same number through DHCP. If you do not have DHCP enabled on your router, you do not need to worry about this and can simply choose an IP address that does not conflict with your other devices and which is available through your router.