I used the new installer (beta) to install a stable base version. I
installed no extra packages at that time, because I was going to
install the necessary packages later on with apt.
I've read some sites about apt pinning and added the testing and
unstable repositories to my sources.list file.
After some forcing and trial and error, I know this is not the way to
do it, but I still don't know what is. Can anybody tell me what I did
wrong and what I should do?
Apt-pinning is considered harmful unless you know *exactly* what
you're doing. Otherwise, use backports and let apt do it's job.