As of 2014, this website describes the way to do it for Ubuntu and Debian: https://wiki.postgresql.org/wiki/Apt
Copy paste (2014-06-06) in case website goes offline, whatever happens:
PostgreSQL packages for Debian and Ubuntu
The PostgreSQL Global Development Group (PGDG) maintains an APT repository of PostgreSQL packages for Debian and Ubuntu located at http://apt.postgresql.org/pub/repos/apt/. We aim at building PostgreSQL server packages as well as extensions and modules packages on several Debian/Ubuntu releases for all PostgreSQL versions supported.
Currently, we support
Debian 6.0 (squeeze), 7.0 (wheezy), and unstable (sid) 64/32 bit (amd64/i386)
Ubuntu 10.04 (lucid), 12.04 (precise), 13.10 (saucy), 14.04 (trusty) 64/32 bit (amd64/i386)
PostgreSQL 8.4, 9.0, 9.1, 9.2, 9.3, 9.4 beta
Server extensions such as Slony-I, various PL languages, and datatypes
Applications like pgadmin3, pgbouncer, and pgpool-II
Packages for older PostgreSQL versions and older Debian/Ubuntu distributions will continue to stay in the repository; updates for those will be provided on an ad-hoc basis.
Create /etc/apt/sources.list.d/pgdg.list. The distributions are called codename-pgdg. In the example, replace wheezy with the actual distribution you are using:
deb http://apt.postgresql.org/pub/repos/apt/ wheezy-pgdg main
(You may determine the codename of your distribution by running lsb_release -c.)
Import the repository key from https://www.postgresql.org/media/keys/ACCC4CF8.asc, update the package lists, and start installing packages:
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install postgresql-9.3 pgadmin3
Alternately, this shell script will do the above steps for you.
9.4 beta only: See FAQ on beta releases
Have a look at the FAQ.
Note: This repository provides "postgresql", "postgresql-contrib", and "postgresql-client" meta-packages that depend on the latest postgresql-x.y, ... packages, similar to the ones present in Debian and Ubuntu. Once a new PostgreSQL version is released, these meta-packages will be updated to depend on the new version. If you rather want to stay with a particular PostgreSQL version, you should install specific packages like "postgresql-9.3" instead of "postgresql".