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

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";