I'm not sure what age level you are refering to. All of my students in grades 3-6 do a 5 minute drawing in their sketchbooks upon entering the room. This segment of "Silent Sustained Sketching" is based purely on observation. One technique I use is a viewfinder, to "frame" the still life objects so that they can see how the shapes relate to the edges of the paper. Students are instructed to keep their heads still so that the view doesn't change. We practice every class and I add techniques each year as they progress. Blind contour line drawing and upside down drawing help them to be more observant. After discussions about the picture plane, I also use sheets of plexiglass and dry erase markers and let them trace the setup of the still life when they are having a hard time with it. Sometimes, just that will give them a deeper understanding of how a line "goes". By 6th grade I cut apart a photo and they each draw an unidentifiable piece and we put it together. They are amazed at themselves and by this
time agree with me that drawing truly does begin with "SEE"! Then we tackle a self portrait and the results are always awesome. By the way, when someone insists that drawing from their imagination is what some of the famous artists do, I break out my Picasso power point with some of his early portraits and let them see that he was a MASTER at observation before he progressed into his cubist period!