The first question, of course, is: what kind of map are you making (and for what purpose)? What scale? How will you use it?

A common way is to leverage shading for a theoretical light source. Brighter highlights and darker shadows indicate stronger correlation with the light source. The notion of ambient occlusion (basically shadows close to the base of things caused by blocking out part of the light from the skydome) can be included by showing lower elevations closer to the edge as darker. Having explicit directed shadows in addition to ambient shadows and basic shading can also help. This technique assumes that you're generating a map where shadows of any sort are appropriate, of course.

Another option is to use contour lines (simple lines or layers that look like the layers in sedimentary stone) that are closer together. The closer together the contour lines are, the steeper the surface (a vertical cliff face has all of the contour lines collapsing together). You can combine contours lines with shading if appropriate to your map to reinforce the ideas.

Hachures are a technique once popular, but they have generally fallen out of favor for most mapping.