Dry bags. Never go on any outdoor trip without them. There are a few things sacred to any outdoorsman- one of them is keeping your gear dry and organized. On our treks, Andy and I have tested out a few brands, the best and most durable were Sea to Summit. We used two different types depending on their function.
To keep things organized in our packs, and make sure no water got through to our clothing, sleeping bag, or stuff we didn't want to get wet, we used the Ultra-Sil sacks. These were super lightweight, and completely water proof. It allowed us to separate everything in our packs, so we knew which color bag contained which piece of gear. That organization made camp set-up a breeze.
For our food sack, and side bag (clipped onto our packs and held our guide book, camera, and any other readily accessible gear) was the Lightweight Dry Sack. These were a little more durable and held up to more abuse. These were perfect for our food and anything that was exposed and being hit a lot. It also has a white interior, so it's easy to see all the contents. While not bear-proof, anything waterproof is smellproof, so it made for a great food sack.
Nothing is worse than getting into a camp after a long day in the rain, and pulling out your clothes and feeling that they're damp. Its crucial (not only for safety, but mental stakes) to have something warm, dry, and comfy after a long day -- so my word of advice to you is never leave home without a system of dry bags. These Sea to Summits are tried and true, and also don't weigh much.
For all those gram counters out there, check out the Granite Gear Uberlight CTF3 dry bags... they weigh less than an ounce (even the 18 liter one!) and are supposed to be twice as strong. I have yet to try them out, but they're definitely next on the list!