http://cg.tutsplus.com/tutorials/blender/introduction-to-smokefire-simulation-in-blender-2-5-day-4/

In the next part of this invaluable in-depth series, Gottfried Hofmann, the man behind the official Blender smoke/fire documents, continues his look at smoke and fire simulation techniques within Blender 2.5. The 4th part of the series is all about working with optimized domains to improve your hi-res simulations. Prepare to be amazed at just what Blender can do for free!