I borrowed an idea from ancient mariners who used a distance finding device called a kamal - it's a notched stick with a string that has a knot at it's end which is held in the teeth...this keeps the kamal a fairly precise distance from the eye. I use a piece of plastic with a 4x5 hole cut in it that has strings of several lengths hanging from it, long for long lens, short for wide lens. These are knotted so when held in my teeth, I see what the film see's when the camera is focused at infinity. By noting what's in the bottom corners of the frame, I can, for example, know exactly when a cloud is just nestled in the top left corner of the frame when the dark slide has been removed. It also helps in isolating a composition from it's surroundings to see if it's worthy.
I used to only take one image from any one subject matter - walking around, back and forth, moving up and down until I found THE strongest composition. I've lightened up in the last few years as I realize I could walk for hours before finding another equally strong subject, and will now spend some time exploring several different compositions.
I try to have no preconcieved ideas about what I'm to photograph...they box me in and may make me miss a subtle composition that's just barely whispering to be seen.