The human eye seeks the light, and usually locates the brigthest spot in the image. If there is one bright spot, that's usually the place we start looking.
There are no definitive rules in photography, and I'm not trying to say that you always need to let the subject be the "bright spot" in the image, but if you want to lead the viewer to the most important part immediately, you should make it be brighter than it's surroundings.
When we look around our eyes tend to follow lines and connected "paths". There are unlimited ways of how to create such paths, but try using "lines" in the environment. It can be tree branches, buildings, roads,...anything that seems to be connected, but not cluttered. Something with contrasts, that's easy for our eyes to identify and "follow around".
I really enjoyed Michael Freeman's book The Photographer's Eye, in which he explains this topic well.
To illustrate how our eyes search bright spots, and follow lines, I have linked to a couple of images from my Flickr page.
Here is an image from the St. Peters church. What is the first you notice in the picture, and how far does the eye lead you automatically?
Bright spot in St. Peters Church
Take a look at the image of the bridge in the clouds, and notice what parts of the image you see by letting your eyes guide you automatically.
Leading lines on Bridge in the clouds image
When I'm looking at the picture from St. Peters Church, my eyes goes directly to the window, and then following the light down to the group of people. It's no path for the eyes to follow from there, which actually works in this picture: For me the most interesting thing is what's happening just there where the light hits the people. ...and who is that enlightened person which everyone is gathering around? ;)
In the other image, the bridge it self is the main subject. There are not much contrast in the image, so my eyes does not lock on a specific parts of the image. Instead the eyes "grab on" to something and follows the lines around the picture from there on. When I'm viewing this image, my eyes basically explores the entire frame, ending up in the center of the image where all the lines end.