1. Step

add testing repository to /etc/apt/sources.list;

# vim /etc/apt/sources.list;

deb http://ftp.arnes.si/pub/packages/debian/ testing main

deb-src http://ftp.arnes.si/pub/packages/debian/ testing main

2. Step (example for packages wordpress + tinymce)

# vim /etc/apt/preferences;

Package: *

Pin: release a=testing

Pin-Priority: 300

Package: wordpress

Pin: version 2.*

Pin-Priority: 700

Package: tinymce

Pin: version 3.*

Pin-Priority: 700

Default priority is 500.

Greater the number higher the priority

3. Step

# aptitude update && aptitude install wordpress