I purchased a Carradice Camper Longflap a few months ago and I love it. It's like having a third pannier on your bike that seems almost infinitely expandable. Before Xtracycle's near perfect Free Radical bags, I'd say that Carradice Longflap and its smaller brethren were the epitome of bike luggage.
This particular model is a "longflap" because it's outer straps can expand to accommodate more stuff! Like a sleeping bag in this case. It also has handy D rings on the outside that allow you to strap things down (like a folding tripod chair, or say a sweater or a bottle of water).
I usually put things I would access fairly often on a tour. Things like food or a light sweater or shirt and some tools. I purchased mine from Peter White Cycles but you can also get it from Wallingford Bicycles. I've purchased things form both merchants and they have been great to work with.
Still Snowing in the Methow Valley
14 hours ago