I'm currently writing a script which needs to parse a configuration file and for one reason or another I got in to the situation where I wanted awk to use one of my shell variables.  I had never needed to do this before so I searched for a little while before coming across a guide on Tek-Tips that explains three ways of doing this.

http://www.tek-tips.com/faqs.cfm?fid=1281

This link in now in my favourites and I'm sure I will come back to it again and again.