There's not much you can really do as far as periodic maintenance with the bearings since they are sealed units. The biggest thing is just keeping them clean and avoiding high pressure washing or use of degreaser on them. You guys are in the PNW right? Lots of wet, muddy conditions would warrant pulling the lower link out periodically and giving the entire assembly a thorough cleaning (pins, link, clamping surfaces). Just doing this and having a fresh coat of anti-sieze might be all it takes to cure the creaking. Another spot to check is the shock mounts. We apply a thin coat of grease to the reducers on the shock before installation and that really helps keep things quiet. As for what constitutes "periodic" intervals, that will depend on how muddy things have been. Hope this helps!