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add this to /etc/apt/sources.list (replace the mirror with one next to you if you want):

deb ftp://ftp.de.debian.org/debian/ unstable main non-free contrib
deb-src ftp://ftp.de.debian.org/debian/ unstable main non-free contrib

replace 'etch' by 'testing' in all other lines

add this to /etc/apt/apt.conf.d/01local (the name of the file does not matter, as long as it's in the same directory):

APT::Default-Release "testing";