Originally Posted by markf
A silk liner will add warmth and keep the bag clean, so you just wash the liner instead of the bag. Wearing a hat to bed will help trap heat inside your body, since so much heat loss is through your scalp. Your choice of ground pad will make a difference too, a thicker pad or a second pad will prevent conductive heat loss into the ground. Keep in mind that a bag that is too warm will make you sweat at night, leaving you dehydrated when you wake up in the morning.
All very good advices and suggestions. You can also further boost the temp rating if needed by sleeping in thermal underwears, , mittens, warm socks and maybe a down vest. I've used a 0 Celcius (freezing point for water) rated sleeping bag in much colder temps using those methods. My "secret" weapon which is in a league of its own for adding warmth at minimum weight is a Psolar balaclava with heat exchanger -
www.psolar.com. Lots of people are overly scared of being cold and end up sweating through most nights in bulky, heavy and expensive sleeping bags. Get a light and compact down bag and fine-tune with internal and external additions. That's how I do it anyway.