Suspension seat post for the rear means you don't need to call out the bumps so much. It also means that you don't interrupt the flow and need to call to coast. I picked a SunTour SP12 NCX as it was an affordable parallelogram linkage seat post - piston type seat posts are more prone to lateral and rotational slop as the bushings wear out, and the Redshift and Cane Creek Thudbusters were about 2-3 times the cost. Just make sure you set the sag and preload on it for your stoker.
That and positioning the saddle and reach as previous posters have mentioned (basics, should be done regardless).