Many suggest that students should first learn how to estimate the slope of a graph by hand before using a least squares linear regression routine. Students must first draw a straight line through their data that represents the linear trend of the data. They may want to draw three lines for: the best guess; the largest slope; and the smallest slope consistent with the data.
The links below provides basic guidelines for estimating the slope of a line.
Slope of a Straight Line is from Purple Math. This site is intended as an algebra review/tutorial but it is useful in that it has interactive practice examples for students to test their understanding.
Determining the Slope on a v-t Graph is from a physics class using velocity time graphs. The practice and self-check activity with this tutorial can help students avoid common mistakes.
This Power Point Presentation on Graphs in Economics (more info) from Rutgers University has some good material. Slides 22 -24 are particularly relevant and may be of use to you when discussing slope calculations.