Drupal Upgrade Guide/Tutorial

Drupal Update Guide:
 
Back up site/ftp
 
Back up/export database - phpmyadmin
 
Note which modules you are using
 
Update modules to current versions for current drupal version prior to core drupal upgrade
 
Check to see that your modules are supported in next drupal version
 
Go to site status page
Run cron
Update database if necessary 
 
Switch site to default theme
 
Turn off clean urls
 
Disable all performance/caching
 
Turn off modules
 
Put site in standby/maintenance mode
 
Upload new drupal version files to server
 
Upload your old: 
.htaccess 
Files directory
Sites/default/settings.php
Any custom scripts, subdomain folders, custom themes
 
Go to yoursite.com/update.php
 
Download new versions of your modules then upload them to your site
 
Update previous custom theme to work with new version
 
Turn everything back on in reverse order
 
To be extra careful, update the database (update.php) after each set of modules you enable - you can check if its necessary on the status page.
 
COMMON PROBLEMS:
permissions settings on directories
file system settings
image toolkit settings
rebuild access permission (if things are missing)
rebuild menu