Forcing static address in 12.04

[rikona]

I had to replace a failed router in the lan, and needed to temporarily
set a 12.04 box to dhcp. It had a fixed IP since installation. After
the router was configured, I tried to set it back to fixed, but can't.
The network window(s) let me set 'manual' to get a fixed address, BUT
the save button gets grayed out and the fixed IP can't be saved.

I saw a couple of suggestions on the net to change the 'interfaces'
file but I can't get that to work either. Error: 'interface eth0 not
configured'.

The new ASUS RT-N65R router doesn't cooperate either. :-( I thought
I'd just set it to assign a FIP, but it tells me the mac address has
already "been seen" with another IP address (the dynamic one) and
refuses to let me assign a FIP to it. I waited **2+ hours** on hold to
the suggested ASUS tech support line and finally gave up. Most of the
boxes FIPs are set by the router/mac address, and I'd prefer to do it
there if possible.

How can I force a change to a fixed IP in 12.04?

Thanks,

  rikona