Example for Debian Wheezy with libraries from testing:

• /etc/apt/preferences.d/pinning:

  Package: *
  Pin: release a=stable
  Pin-Priority: 700
  
  Package: *
  Pin: release a=testing
  Pin-Priority: 650
  
  Package: *
  Pin: release a=unstable
  Pin-Priority: 600
  

• /etc/apt/sources.list: If it looks like this:

  deb http://ftp.de.debian.org/debian/ wheezy main contrib non-free
  deb-src http://ftp.de.debian.org/debian/ wheezy main contrib non-free
  
  deb http://security.debian.org/ wheezy/updates main contrib non-free
  deb-src http://security.debian.org/ wheezy/updates main contrib non-free
  
  deb http://ftp.de.debian.org/debian/ wheezy-updates main contrib non-free
  deb-src http://ftp.de.debian.org/debian/ wheezy-updates main contrib non-free
  

• Add this (make sure the hosts are the same): /etc/apt/sources.list.d/testing.list:

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

• Update and install libraries:

  apt-get update
  apt-get -t testing install libc6 libc6-dev libc6-i386 libc6:i386 libc6-i686:i386 locales
  

Sources:

• ​http://jaqque.sbih.org/kplug/apt-pinning.html
• ​http://stackoverflow.com/questions/10863613/how-to-upgrade-glibc-from-version-2-13-to-2-15-on-debian