In Migrating from Joomla! 1.5 to Joomla! 2.5, Jen Kramer walks through moving a typical site to Joomla! 2.5 using two different tools, jUpgrade and SP Upgrade. jUpgrade and SP Upgrade migrate the 1.5 database content to 2.5 automatically, which maintains your data and settings. Jen shows how to download and configure the tools; how to adjust modules, images, and templates that may have shifted during the move; and how to update an old custom template for Joomla! 2.5 and fix any styling issues postmigration.
Note: jUpgrade is free, while SP Upgrade has a small fee.
Understanding the differences between Joomla! 1.5 and 2.5
Talking to a client about the migration, plus bulletproof reasons why they'll want to move their site
Downloading, installing, and configuring jUpgrade and SP Upgrade
Reassigning modules to their correct positions
Fixing page headers
Reinstalling third-party extensions
Correcting custom templates' XML and index.php files