The rear delts usually get neglected, but this is a pretty easy fix and one you can do at home on a daily basis. The band pull-apart will bring your sad little finger-sized rear delts up to snuff quickly.
You don't need a heavy-duty resistance band because you're going to keep the reps high. Rock bottom rep range is 20, done for 4-5 sets daily.
You multitaskers out there will love this movement as it allows you to work the rectus abdominis, the obliques, and the serratus at the same time. Too bad the rest of your life isn't as efficient.