I'm struggling to get an equation working in QGIS that I've seen posted for ArcGIS. This article has a full description. Long story short, the author modifies the original Tobler's Hiking Function for use in ArcGIS, but it won't work in QGIS Raster Calculator. I've spent a few hours working on this with no luck.
The original formula is as follows:
(your_res /1000)/(6 * Exp(-3.5 * Abs(Tan(("your_slope" * 3.14159)/180) + .05)))
"your_res" = resolution/pixel size of DEM (in this case, ASTER) "your_slope" equals slope raster (calculated using Raster -> Analysis -> Slope).
Here's how I modified the equation for QGIS Raster Calculator:
(your_res / 1000) / (6 * 2.71828 ^ (-3.5 * abs ( Tan ( ( "your_slope" * 3.14159)/180) + .05)))
The resulting raster returns few values and basically flattens the image out. Only the most severe slope changes show any variation. I don't have ArcGIS at home to test, but that article is fairly well known to archaeologists and I suspect it works.