Just a quick dump of links that I searched to get a handle on IDL's syntax for formatting strings.
These two may have been the best pages, with examples that show the nearly but not quite C-like formatting methods and a nice table of the format codes:
http://idlastro.gsfc.nasa.gov/idl_html_help/C_printf-Style_Quoted_String_Format_Code.html
http://star.pst.qub.ac.uk/idl/Format_Codes.html
This one showed one example of how to print an array to a file with formatting:
http://www.cis.rit.edu/class/simg211/datao.html
For a beginner's guide, this page was surprisingly useless:
http://www.stsci.edu/hst/training/events/IDLTopics/SSD98IDL/IDL_dataio.html#Writing%20Formatted%20Data
Here's the whole damn manual:
http://www.astro.virginia.edu/class/oconnell/astr511/IDLresources/getting-started-IDL-v7.0.pdf
