For me, the place has to be right. Luckily, there are a lot of values of 'right'. My workspace at home is in the bedroom I share with my husband. We sleep in a loft bed, and my desk is underneath. There are twinkly lights strung up below the mattress, so I have nice, soft lighting. My desk is set into our big bay window which looks out onto the park. I need to be able to look at something that's not a computer screen sometimes. It's nice to look out and see traffic going by on one side and the trees swishing in the wind on the other, nice to see the reflections of the lights in the window. The other place I spend a lot of time writing in is a local wood: I take my laptop there, have endless cups of tea at their outoor café, and write in a setting very much like the place my current project is set in.
Music is important. I have a playlist of music that inspires whatever I'm working on, and I keep that on shuffle on the mp3 player or the computer itself while I'm working.
I find that being in a physical space that says, 'this is a writing space' to me nad having the right music helps pull it all together to the point where I can usually just dig right in and start writing. I work part time, three days a week, so my writing times are the two days I do not work, weekends, late nights, and occasionally lunch breaks at my day job (where there is a garden that is a lovely space for writing). On days when I'm writing at home, I get up, get husband and kid off to work and school, make myself a cup of tea, and get to work, just as I would if writing were a full-time occupation. I start with any research tasks I need to accomplish, then make notes and get writing. I don't turn the phone off or refuse to answer the door, and I tend not to use headphones unless everybody else in the house is asleep. When I'm writing at the wood, I get out of the house as early as I can, make sure the writing music is cued up on the iPod, and listen to music and think forward to what I'm going to write that day on the short bus journey to the wood. When I get there, I start with a walk, eventually arrive at the café where there are tables, chairs, sometimes scrambled eggs on toast for breakfast, and I write for as long as I can there, three or four hours. I finish up that day with a walk through the wood to the bus stop for my journey home.
Maybe it sounds a little routine, but I find the ritual of being in the right place physically helps quite a bit with my journey to the right place mentally.