In reply to adam11:
A silk balaclava (and liner socks) is a lightweight alternative to a silk liner for keeping the bag clean... The balaclava keeps the face oil off the bag, and the liner socks keep smelly, sweaty feet off the bag; the two places where a sleeping bag gets dirtiest, assuming you're wearing long base layers...
Admittedly, you do look a bit of a nutter...
I bought silk balaclavas dead cheap from www.silkbalaclavas.com a couple of years back. Hmmm... They want £10 now. I paid £7.50 for two...
I got my silk bag liners from Lidl; £8, IIRC.