Once you delve into the ArcXML Programmer’s Reference Guide you notice that the cartographic capabilities of ArcIMS are increased, when compared with those offered in through ArcIMS Author - ArcMAP provides a solid UI to define your map service symbology, whereas to take advantage of the symbology options not available in ArcIMS Author, you must update the ArcXML code directly (using a program like TextPad).
As ArcIMS also supports raster marker symbols, which aren't available in ArcIMS Author, I have found the following script to be infinitely useful:

  http://arcscripts.esri.com/details.asp?dbid=12766

This script outputs an AXL file based on an MXD. Note I have found that some of the symbology does not carry across entirely, so if using this utility you will likely have to do some tweaks (sometimes polygons receive polyline symbols...) - I find that this tool is especially great for theming different classes of polygons and symbolizing point layers with the ArcGIS symbols.
By the number of downloads recorded, I am not the only one who finds this script invaluable – thanks Jeroen!