I don't like the one-big-garbage-bag-inside-the-pannier option as it's hard to pack. We just packed our stuff in normal plastic grocery bags and then put those bags in the panniers. We end an endless supply of grocery bags (except in some places in Argentina where they were banned!) so when they started wearing out we just got new ones. The stuff that we didn't use often got stashed in a grocery bag at the bottom of the pannier and the stuff we used a lot was at the top. It was easy.

Ziploc bags are great, but they do wear out over time. We found that, after a couple years on the road, that we didn't use Ziplocs at all - had some ditty bags and then used plastic grocery bags to keep them dry.