What is the recommended way to install GIMP 2.8?
The version present in the default elementary Luna repositories is 2.6.
The recommended way would be to upgrade to Freya, sooner rather than later. That will get you a whole host of updates on various things, including updating GIMP to 2.8.10.
With a little command-line magic, you can indeed upgrade GIMP, and only GIMP, in a way that will keep you up to date in the future. Otto Kesselgulasch's repository will likely upgrade you to the latest release, currently 2.8.14.
The code below adds and auto-confirms the PPA repository, and quietens down the updating a little. That means it may take a while to show progress, and may look like it's stopped, but rest assured it's still working away.
sudo add-apt-repository -y ppa:otto-kesselgulasch/gimp && sudo apt-get update -qq && sudo apt-get install gimp