I use a pannier rack on my regular commuter and my back-up bike. I've got a grocery pannier that I use for my ten cent garage sale briefcase. I, sometimes, just use my small canvas knapsack that I bought from Goodwill for like three bucks. I've got a framepack that I bought from Goodwill for 99 cents that I keep on the bike all the time for little stuff that I want to make sure I've always got.

Seems the more space I have available, the more crap I seem to "need." So, I try to keep it to a minimum. Whatever I carry on the bike, I carry indoors at my destination. Therefore, I try to keep it to a briefcase or the small backpack.