I have a winter down bag and do much the same as above. Being a winter bag, it doesn't get much use, but it hasn't been cleaned in over 10 years from new. Every time I use it I make sure that my skin doesn't touch the inner. I hate bag liners because I'm a bit of a wrestless sleeper and get all wrapped up in them, so I just wear lightweight clothes in the bag - socks, longjohns, baselayer and a thin beanie on my head. All lightweight clothing I would be wearing anyway, the next day or for emergency backup, so no extra weight in my backpack. The bag does smell a little bit now, but it is not dirty inside.
Basically, it's the bits of your body that rub most on your sleeping bag or get the most smelliest. So, protect it from your feet, backside, back, shoulders and greasy hair and it will last longer between needing washes.
Edited: 29/08/2007 at 23:41