You might find this thread useful/interesting. It also mentions the vertical central crease.
http://translate.google.co.uk/transl...n&hl=&ie=UTF-8
This is a cruder method, shows the crease on this one.
http://translate.google.co.uk/transl...n&hl=&ie=UTF-8
If these links don't work due to being through Google translate, go here and use Google translate to German if needed. Both of these threads are in this section.
http://amazon.forum.bilia.se/phpbb3/...2698aaa1aa7a0b