*note i still prefer aptitude over apt-get

easiest way switch to testing:

sed -i 's#jessie#testing#g' /etc/apt/sources.list aptitude update aptitude dist-upgrade

or just install the xfce packages from testing(should work but havent tried that since i want testing security updates):

printf "deb http://ftp.us.debian.org/debian/ testing main" >> /etc/apt/sources.list aptitude update aptitude install -R xfce4/testing

the -R will also not install package recommends(see man aptitude)