Export Modelsim waveforms as image for printing
in a form where they can look decent when printed


is a very subjective statement. Moreover, you can't really customize MSIM wave editor to display things very differently. Considered your question is quite broad, the simplest way I can think of is to import your wave in a waveform viewer you like better.

To export the waveform in a commonly usable vector format you can use the command line tool from vsim (2005 onward):

&gt; wlf2vcd
&gt; Usage: wlf2vcd [&lt;Options&gt;...] [-o &lt;outfile&gt;] &lt;wlffile&gt;


Which will convert your selected *.wlf file into a common value change dump file, holding all the informations you need.
(more on that https://en.wikipedia.org/wiki/Value_change_dump)

The source waveform (.wlf) file is commonly generated by any modelsim simulation and saved, when you exit vsim, under vsim.wlf unless another name was specified.

Once you have your waveform in .vcd format, you can import it in any waveform viewer of choice and style its display according to your preference.

You can check for viewers in https://en.wikipedia.org/wiki/Waveform_viewer , or you can even code your own, provided you understand VCD syntax.



Alternative

Unless you have to deal with plenty of waveforms, objects and processes, and instead make a functional waveform focused on a specific behaviour, I would reccomend using a simple wave editor to display what you need to.

My personal favourite when I have to export waveforms I analyzed to colleagues which wouldn't benefit from MSIM view, I go to http://wavedrom.com/ which is quick enough to make an effective (and -IMHO- pretty) graph.