I've found that carrying a (oh, dare I say it) digital (whew, that wasn't too difficult) camera is perfect for those times when I'm just not sure of a location, or of the lighting. I can set the exposure manually (Canon S30) and set other camera settings to duplicate my film the best I can and grab a shot to see how it looks on that tiny screen (zoomed in, of course). It's my sketch pad, and my 4x5 is the canvas.

And if I want to try the experimental stuff, I can get digital shots of it just for my website, and save the film for the stuff I want to print.