An effective slideshow isn't just a series of slides; it should have a definite beginning, middle, and end. In other words, there should be distinct groups of slides focused on specific topics or themes.
The Navigator can be hidden or revealed by using its option under the View menu item. If you move the thumbnail of one slide to the right, it becomes a "daughter slide" to the slide above it. Daughter slides can be toggled in and out of view by flipping the small black triangle that appears next to the "parent slide" above it.
A common approach is to create a parent slide with a sub-sectional title and then place a number of daughter slides bearing the actual content for that sub-section underneath it. At its simplest, you might have an introductory sub-section framing the questions being discussed in the presentation, another containing your data, and then a third summarizing your conclusions.
New slides inherit their position in this hierarchy from the slide immediately above them in the navigator. However, by pulling new slides to the left or right in the navigator, you can change their position, turning a daughter slide into a parent. You can also move slides from one group to another, or, by Option-dragging, duplicate them.
Figure 5: Use the Navigator sidebar to arrange slides into groups by "indenting" them. Nest these as much as you want, and to keep things simple, use the "triangles" to hide groups of slides you aren't working on.