edit: Manual installation and use of setuptools is not the standard process anymore.

If you're running Python 2.7.9+ or Python 3.4+

Congrats, you should already have pip installed. If you do not, read onward.

If you're running a Unix-like System

You can usually install the package for pip through your package manager if your version of Python is older than 2.7.9 or 3.4, or if your system did not include it for whatever reason.

Instructions for some of the more common distros follow.

Installing on Debian (Wheezy and newer) and Ubuntu (Trusty Tahr and newer) for Python 2.x

Run the following command from a terminal:

sudo apt-get install python-pip 

Installing on Debian (Wheezy and newer) and Ubuntu (Trusty Tahr and newer) for Python 3.x

Run the following command from a terminal:

sudo apt-get install python3-pip

On a fresh Debian/Ubuntu install, the package may not be found until you do:

sudo apt-get update

Installing pip on CentOS 7 for Python 2.x

On CentOS 7, you have to install setup tools first, and then use that to install pip, as there is no direct package for it.

sudo yum install python-setuptools
sudo easy_install pip

Installing pip on CentOS 7 for Python 3.x

Assuming you installed Python 3.4 from EPEL, you can install Python 3's setup tools and use it to install pip.

# First command requires you to have enabled EPEL for CentOS7
sudo yum install python34-setuptools
sudo easy_install pip

If your Unix/Linux distro doesn't have it in package repos

Install using the manual way detailed below.

The manual way

If you want to do it the manual way, the now-recommended method is to install using the get-pip.py script from pip's installation instructions.

Install pip

To install pip, securely download get-pip.py

Then run the following (which may require administrator access):

python get-pip.py 

If setuptools is not already installed, get-pip.py will install setuptools for you.