how to carry my telephoto lens? In a lens case, or bag?

I just finished a long SEA trip with a Pentax K100D, 18-55, and 50-200. I used a Tamrac camera bag. Worked for me.

The model that I got has a top flap that folds back behind the bag so that it's out of the way. That turns the bag pretty much into a camera "holster" that holds the camera with either lens in the center pocket. Just whip it out when I need it.

When the going got rough I could hold the bag in my lap to shield it from bumpy rides.

Shooting from a backpack is a pain. In my experience....

(BTW, I found that I kept the long lens on most of the time.)

I think the answer is in the question, particularly when you mention roughing it in Tibet. If the lens is important enough to bring along then you have to protect it from collisions and atmospheric realities. In your backpack the lens should be packed in whatever clothes you can wrap around it, not hangin out in an exterior pocket. You will still know where it is and be able to grab it for those special shots. Better yet would be a camera bag that contains all that you need. When you head out for a lookabout, it's useful to have a camera in your hand and all the other necessary gear hanging ready off your shoulder.

I'd buy a holster bag as mentioned by #1 that fits your camera and 14-45 lens. Most of your shots will be with this lens. Buy a separate hard lens case for the 40-150.

When in situations where you might want to shoot, keep both handy. When not, keep both buried in your pack for protection against bumps and drops.

Actually I didn't suggest a holster. I use a "normal" camera bag and stick my camera in the center section when not in use.

(Never did like neck straps. Use a piece of sturdy cord and plastic toggle as a wrist strap.)

thanks for the suggestions everyone, I will try it out accordingly.