It's easy! Just buy a one pound bag rated to -30 degrees, only costing two bazillion bucks. Maybe you have a cheaper bag, just getting into backpacking, and are interested in ways you can still get a good night's sleep without freezing. Here are some techniques and I am sure others can suggest others or improve on these:
Location. extremely critical. Avoid cold air drainages and spots where cold air can collect. Stay away from the bottoms of drainages and look higher upslope. Avoid windy locations, sometimes by seeking out brushy locations. You may wish a spot where you will get early morning sun (or not).
Use a bivvy sack, a properly pitched tarp, or a tent to keep things 5 - 10 degrees warmer.
Use clothing, either worn, or properly placed within your bag or the bivvy sack. Be sure you do not compress the insulation within your bag. Make sure stuff is as dry as possible.
insulation beneath your bag is critical. This may be best location for clothing or similar stuff. consider a layer of dry leaves, etc. which might be handy.
It pays to eat a hearty, nutritious meal before sleeping.
A warm, water filled bottle is often tucked in bags, especially to warm feet. Just be sure the bottle is leak proof!
Huddling together, even within the same bag or shelter can be effective in reducing heat loss. just be sure everyone is agreeable to this solution, since social conditions can vary considerably.
1) Get out of damp clothes. Even the little bit of residual moisture from sweating can decrease the efficiency of insulation. I try to keep one set of long underwear absolutely dry, to be worn at night only, and change into it just before crawling into my bag.
2) Drink something hot shortly before bed, preferably something with no caffeine. Herbal teas or even hot water will do.
3) A hat and socks (dry, and not tight at all on your feet) help immensely.
4) The more you move around, the colder you will be, so get situated as best you can, as quickly as you can. Something under your mattress doesn't feel good? Fix it! Shirt not comfortable? Change shirts!
5) Be sure nothing is touching the side of the tent.
After dinner heat up some hot water and fill up a plastic water bottle. Now stick that into a sock and put it at the bottom of your sleeping bag. The sock prevents you from burning your feet on the hot bottle-because it will be hot! Your sleeping bags insulation will keep it hot for many hours and help you to get to sleep faster and stay asleep. Be sure to wear a wool-polyester stretch cap at first even if you are warm. It is easier to stay warm rather than going to sleep just comfortable and than trying to warm up after you are already cold.
Know your conditions, and the possible conditions before you go.
The last kayak trip I took was in the PA grand canyon. Day temps in the town were 60's, night temps were 40's. In the gorge it was <28.
I took a 30F bag and a fleece bag. I ended up with the fleece as a liner and the 30 on the outside, nice and comfy. Had a knit hat (ALWAYS) the only thing that got cold was my nose and I woke up to ice on the tent.
Chem handwarmers in the bag can help with cold feet.
Second the long johns. Not so much the socks.
One more thought - Give your ag plenty of time to expand fully when you are making camp. The sooner it is out of its bag, the warmer.....
I find I sleep warmer in cool and even cold conditions if I wear a dry pair of socks in my bag. In colder temps, I wear a dry base layer coving both legs and arms. In really cold conditions, I wear a head covering. I reserve the sleeping socks and base layer as my emergency clothing in case I get really cold during the day or evening, but generally try not to wear them unless absolutely necessary except in the sack.
High rated R value sleeping pad. And don't overdo it on the layers of clothes, especially don't wear your rain gear to bed. Learned all this the hard way and wondered why I was always so cold.
Your rain gear isn't breathable.. no matter what the companies try to tell you. It traps your heat in. You'd think that was good, but it's not. Your bag works by using your body heat. If you have on too many layers or rain gear that prevents your body heat from getting to your bag/quilt, you'll stay much colder.