Paths

This important bit is already covered in the IoA guide to computing systems
The easiest way is just to follow the instructions and source the idl setup file. You need to make sure the enviroment variable IDL_PATH is set:
echo $IDL_PATH
If not, you need to add a line to your shell startup files. Find out which shell you are using:
echo $SHELL
In bash, add something like this to your .bashrc:
export IDL_PATH=+/usr/local/itt/idl
In csh, add to .cshrc:
setenv IDL_PATH +/usr/local/itt/idl
Use + before the path to expand it down the directory tree.
Use : after the path to list more than one directory
In IDL, check what you have in your path:
print, !path
Add more libraries to path using startup file

Startup file

Startup file is, simply, a list of IDL commands that will be executed at IDL start up.
All you have to do is to tell IDL where it is (in bash):
export IDL_STARTUP=my_startup.pro
or in csh:
setenv IDL_STARTUP my_startup.pro
More on configuring IDL

Editing/Developement

IDLWAVE - add-on mode for Emacs/Xemacs
The beauty of IDLWAVE explained
IDLWAVE getting started tutorial
Emacs tutorial
Emacs commands list by topic

¿MATLABLAS IDL? - LANGUAGE DICTIONARIES

Mathematica, Maple, Matlab, IDL translations at amath.colorado
Matlab vs IDL by Michael Liebling (PDF)

First Aid

In IDL type: ?
Periodic Table of IDL operators
Coyote's Guide by David Fanning
IDL/PVWave Google discussion group

Program libraries

IDL Libraries Browser by Eric Deutsch
Coyote Program Library
Astronomy User's Library
Markwardt IDL library - home of mpfit and more
texToIDL - use TeX expressions to annotate plots
Code Library at User Community of whatever the name of the company that owns IDL now

IDL Voodoo

Working with color - a (PDF) chapter from D. Fanning's book
Coyote's IDL programming tips include such pearls as:
  File Naming Conventions
  IDL Operator Precedence: Who's on First?
  How Do Colors Work on a 24-Bit Dsplay
  Array Storage and Indexing
  HISTOGRAM: The Breathless Horror and Disgust
  Array Concatenation
  Dimensional Juggling Tutorial
  Are FOR Loops Really Evil?

Common sense

File Naming Conventions - D. Fanning's guide

Color
Color Tables

IDLWAVE tricks

Compile regions: mark block and say C-c C-d C-e
Commenting out sections - mark block and C-;

Astronomy

FITS I/O with IDL
Galactic extinction by Schlegel, Finkbeiner, Davis
together with the filter table
UMich/UChicago IDL Libraires - SDSS data analysis

Books/Guides

Interpolation - a chapter from An Introduction to Programming with IDL by Kenneth P. Bowman
Writing an IDL Graphics Display Program - chapter 9 from IDL Programming Techniques by D. Fanning