Waterproof bags are essential. Chances are if you go on the road for a long trip you are going to hit some rain. It isn't practical for me to waterproof all my gear but at the very least, I try and protect the things that would be ruined by water.
Map roll across the top, left to right: 5L Seal Line Bag, Sea to Summit 1L Bag, and Seal Line Atlas bag. Seal Line makes great bags in a range of sizes from 5L to 55L. The 5L bag was the first one I ever had and I've really tested it's abilities. It has really held up, never had anything inside come close to getting wet.
Serious fold-over closure, durable materials, very affordable. Highly recommended. I used to ride with a guy who had one of their larger bags and would throw all of his gear in it and just strap it on the bike. Worked great for his style of packing and travel.