Step zero - change you /etc/apt/sources to new release
First the list of available packages for the new release needs to be fetched.
aptitude update
Before any change try dry run
aptitude -y -s -f --with-recommends dist-upgrade
Now you have a chance to eliminate problematic packages, especially when you don't need them.
EXAMPLE: aptitude remove libldap2
Then upgrade packaging system
aptitude upgrade
Finaly upgrade the whole distribution
aptitude dist-upgrade