The ideal camera bag should fit comfortably while allowing you to carry your gear in an unobtrusive way. As such, you'll want to give careful thought as to what system will work best for your individual needs. Since I shoot a wide variety of subject matter, I required a versatile and rugged solution. I also wanted something with a waist strap to take some of the weight burden off my back. Before finding the right bag, my shoulders would ache after long hikes in the woods.
Rather than leaving necessary gear at home to shed weight, I started to research photo backpacks. At that time, I decided on the Lowepro Mini Trekker AW which offers enough room to fit a camera body with attached 400mm lens, plus a 70-200mm, a 100mm Macro, a Flash, various filters, batteries, and accessories. It also includes a rain cover for surprise downpours, padded shoulder straps, and a supportive waist strap. Best of all, it qualifies as a carry-on for both domestic and international flights. After eight years of heavy use, I'm still using the same bag! Lowepro has since refreshed this line with the new and improved Pro Runner AW backpacks.
Along with the backpack, I use a large toploading holster. Since this sits on my hip it's perfect for quickly accessing my primary camera. The attached lens depends on what the most fleeting subject is. For example, if I'm in bear country, the 400mm is attached to the camera in the top loading holster. If an animal suddenly appears I can quickly access it without taking off my backpack and unzipping it as my subject disappears.
I reverse this setup when focusing on landscapes. In these situations I keep my wide angle lens ready while the telephoto is close by in the backpack. In short, the holster is what I work out of, while the backpack allows me to comfortably transport the essentials.
When flying, I never want to check my expensive photo gear for fear that it could be broken or stolen. This two bag system has made air travel much easier. The backpack is my one carry-on piece of luggage while the holster qualifies as a small personal bag. They both fit under the seat, or in the overhead compartments. This system even worked on a small passenger plane in Costa Rica.
My tripod is the only piece of photo gear that gets checked. Since it can be used as a weapon most airlines have policies against bringing then on board as a carry-on. Of course it's an essential piece of gear that can't be left behind. Here's how I transport it:
First, I remove the ball head and pack that in my carry-on photo backpack. Without the ballhead the tripod is a few inches shorter and fits inside a duffle bag. I then pack all of my clothes around the tripod and wrap it with a few sweaters or pants. This extra padding will protect it from being destroyed when it's tossed on and off the luggage belt. Should this bag get lost or stolen, I'll still have all of my camera gear and the clothes on my back.