For images that can feasibly be vector, I use SVG or icon fonts, depending on the complexity. For bitmaps, make them double the width/height and use width:50%. Obvious problem there is that image sizes could be as much as 4X, so I use a lossless compressor like SmushIt.
I'm starting to use 2x images everywhere by default now, even for non-retina(they scale down ok).
Inline images you can just give a width half of the full-size like. background images you can set a background-size to scale down.
I only use the 1x images for background-images in browsers that don't understand background-size (ie)