What is the recommended way to install GIMP 2.8?
The version present in the default elementary Luna repositories is 2.6.
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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