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