i remember there are only 2 kinds of food: Tibetan and Sichuan. try to eat more Tibetan food rather than Sichuan if you can, that means yak every day. get off the tourist track in the centre, get 'lost' wandering around and do not be afraid to enter traditional Tibetan eateries. don't even think about avoiding yak butter tea lol.... it's not all that bad, really.
i was there [and Mt. Everest base camp] in march 2006. a lot must have changed since with the new train.
I was there May 2006 and took the infamous 318 to Sichuan. Much has changed, I was told. Lhasa is very touristy now. I had tea at the teahouse opposite the plaza-shared the table with two monks. There was a nice Tibetan restaurant opposite the Yak Hotel where I stayed. Other choice from Tibetan or Sichuan is Muslim w/ beef noodles and breads. There are rumors about the Tibetan/Muslim animosity so be careful around.