If you use Photoshop, you have to make a new layer, use the "poligonic lasso tool" and create lengthwise, white (or another colour - for example blue if the light source is a blue lightsphere) rectangles, but you have to see to draw them in the right direction (so perpendicular to the light source, but you don't have to paint the light source of course - like Ninjatic). You should also be carefull to make a break between the rectangles, when there is a tree or something, that blocks the light. When you are ready with the rectangles, you have to lower the opacity of the layer, for example to 15% or even 5%, so that the light will not sidetrack from the rest of the artwork. The last step is using the soft eraser with a low "opacity" to erase the endings of the rectangles, so you can create the effect of diffusing light. I hope I could help