I made a simple matboard "viewing frame" to match the perspective of my 7x11 camera and lens. It was a little difficult to use because it depends on holding it at exactly the right distance in front of your eyes. I didn't use it much. What did help was this: I took my 28-70mm zoom on my slr, and noted the focal lengths that match the height and width of the 7x11 camera ( I just set up the 7x11 on my back porch, and then stood next to it with the slr and noted when the horizontal and vertical views matched what was on the ground glass ). Then for a few weeks, when I was out walking, I carried the slr and evaluated different views with it... and like Ian mentioned, eventually I got used to "seeing" the frame without using the slr. These days I just look foolish: I hold my arms out first left/right then up/down and I can get pretty close to what the 7x11 field of view will look like. Over time I've gotten better at holding my arms out at the right angles, and now I can frame a scene quite well.
Sometimes, if there is not much leeway in the view, I still use the slr lens first before I take the trouble of setting up the camera. On multiple occasions, by doing this I've avoided setting up the big camera when it just wasn't going to work. If I'm going to be close to the car, I usually take the slr with me along with the big camera.