The biggest advance in external storage technology in the past few years has been the advent of USB 3.0. While we may see more Thunderbolt-enabled drives in 2013 and beyond, chances are that they will remain expensive, boutique products like FireWire drives before them.
If you buy a USB 3.0 hard drive or flash drive today, you’ll still be happy with it in 2014 and beyond. I do expect external SSDs to get cheaper over the next few years, but they won’t reach price parity with external hard drives for a long time. A relatively new category, wireless hard drives, will continue to improve over the next few years, but won’t match the speed and ease-of-use offered by a simple USB 3.0 drive.