I have camped in Death Valley and in Capital Reef in late November and the temps were frequently mid-teens at night. In South Africa it can be below freezing at night in June or July at higher elevations ( winter for them of course )
There have been a lot of good suggestions here, but I have a couple that I haven't seen mentioned yet -
Get a fleece or silk bag liner - they're small, not too expensive, and really will make your existing bag significantly warmer - fleece is warmer but silk sure feels nice. You can even use one of each if you so choose.
Use a good insulated air mattress or other device to get you off the ground. Very important.
Wear a hat and cover your head and neck with your sleeping bags hood - that's what its for!
Lastly, don't use urine in your bag - yuck!! Do use some nice hot water heated over your campfire or stove - a hot water bottle will really warm up your bag a lot. Put them in your bag 15 minutes before retiring, and crawl into a nice warm comfy nest!
Good safari companies put hot water bottles into your sleeping bags routinely in Africa when it is cold at night.
Also, keep your thermos with you, inside your bag, with some nice warm liquid - hot toddy, or chicken noodle soup, to help you keep warm and comfortable through the night. Or you can eat the soup in the morning before getting out of your sleeping bag in the cold tent.
Then you can wake up and shake all the snow off your tent before getting out of it in the morning.