Short of buying a new sleeping bag, there are quite a few things you can do. Make sure you have a good pad, for one. When you sleep, you of course crush the insulation at the bottom of your bag, so cold can seep up from the ground. You might gain a lot simply by buying an inexpensive closed-cell foam pad ($12 at REI) and laying that over your current pad.
Next, dress for the conditions. For you, I'd suggest trying a full suit of mid-weight long underwear, something like Patagonia's line of Capilene togs ($43 for tops, $38 for bottoms; www.patagonia.com). That stuff offers an amazing amount of warmth with very little bulk. Also, people lose a lot of heat from their heads, and cold feet are never comfortable, so make sure you have a light fleece hat of some sort and some warm socks: SmartWool Light Hikers are ideal ($15; www.smartwool.com) for your dogs; check out hats from Outdoor Research (www.orgear.com), which should have a style to suit to sartorial tastes.
The last step is to add some sort of liner. Design Salt, for instance, produces a line of Cocoon MummyLiners made from a variety of fabrics, including silk and fleece. The silk models (starting at about $50; www.designsalt.com) weigh a mere five ounces and will add nearly ten degrees of warmth. A micro-fleece model ($60) adds about 12 degrees, but also weighs one pound four ounces and is moderately bulky.
If none of that works, then you could consider a down-bag liner, such as Mountain Hardwear's Down Upgrade ($165; www.mountainhardwear.com). It'll zip into most bags with YKK zippers, and can add perhaps 20 to 25 degrees to your current bag's rating. But, in addition to being somewhat pricey, it weighs more than two pounds. So only an option at the last resort.
I think with some adjustments to your sleepwear and pad you'll be fine. And, have a snack before you turn inhalf a chocolate bar, for instance. That'll give you a little extra fuel for staying warm.
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