In most cases you will get away with turning dhcp off on the device lets say satellite box and specify the ip address you want it to stay on as long as its in the range of the router so if you look at the ip address it has now and just specify that with the subnet mask and default gateway it should be fine , failing that look at the ddwrt wiki
http://www.dd-wrt.com/wiki/index.php/Main_Page
found this also
On the Services page, you should see DHCP Server, and underneath it a bar that says Static Leases. Click Add, and it'll open up a space for you to enter client MAC address (left side), hostname (left center), desired IP (right center), and lease duration (right side). The lease duration is in minutes, so for 24 hours you would want to put in 1440. If you don't want the lease to ever expire (meaning the computer never has to refresh the IP lease) use 0.