In August Productions

Ok. So I just looked at two blog posts about MySQL log rotation. While not technically incorrect, I'd like to disagree with the final answer, mainly because recreating the wheel drives me crazy.
Rather than writing a custom script to do this and putting that script in cron, why not use logrotate?
We already ship fully functional log rotate scripts in the Debian MySQL packages. Here's an example:

/var/log/mysql.log /var/log/mysql/mysql.log /var/log/mysql/mysql-slow.log {
        daily
        rotate 7
        missingok
        create 640 mysql adm
        compress
        sharedscripts
        postrotate
                test -x /usr/bin/mysqladmin || exit 0
                # If this fails, check debian.conf!
                export HOME=/etc/mysql/my.cnf
                MYADMIN="/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf"
                if [ -z "`$MYADMIN ping 2>/dev/null`" ]; then
                  # Really no mysqld or rather a missing debian-sys-maint user?
                  # If this occurs and is not a error please report a bug.
                  if ps cax | grep -q mysqld; then
                    exit 1
                  fi
                else
                  $MYADMIN flush-logs
                fi
        endscript
}

This does the re-creation, flush logs and handles failure conditions gracefully. It also compresses old files. Even better - your system already has logrotate running (unless you run a really bad distro)
Now, if you aren't on Debian, don't just copy this verbatim, as you probably don't have a debian-sys-maint user or an /etc/mysql/debian.cnf file. In Debian, we make this at package install time and refresh it on upgrades. It's a system user with an autogenerated password put in a file only readable by root, that allows system scripts like this to run unattended.
Once you read up a little on logrotate, you'll realize it's really great at just about anything you need to do in the log rotation field. Have fun!