Slug's Guide to IDL: A great collection of resources compiled by UC Santa Cruz grad students. A few highlights:

Plotting examples with the new "object graphics" method

IDL Astronomy User's Library: The best thing about IDL is the huge astronomy codebase. If you think it has already been done, it probably has: don't reinvent the wheel! This is a good place to start looking.

The IDL Coyote, aka David Fanning, will surely show up in many of your Google searches. His website has lots of great question & answer archives, tips, and a code library. The Coyote has a specific page on object graphics

NumPy for IDL users: Useful mapping between IDL and Python if you use code written in both languages