Amazing that this still isn't a built-in feature in Illustrator even in the 2014 release. There are a few scripts floating around to do this quickly and easily, I recommend this one from the arcticmill.com folks. Visit their post to download the script (direct link to the .jsx file).
By default it spits out hi, med, and low sized versions and you can customize it as you see fit. Also a great feature it includes is the crop to artboard which was handy in a recent case. Options for .svg, .jpg, and .png are great as well.
It wouldn't take much for Adobe to include useful scripts like these in the default install, much the way the do for Photoshop and After Effects. Not really sure why the scripts shipping with Illustrator have been pretty barren for as long as I can remember.