As long as it's not
too cold, don't worry too much. What you do have to watch out for is cold drafts slipping in through the bars. If you're worried that it's really quite chilly in the room, the advice given so far is good!
In the past I've propped up cages on stacks of books and put a warm (not hot) hot water bottle wrapped in a towel underneath. Don't let it touch the cage, it's good to have a gap of air between the cage and the heat.
Other than that, I have spare low-watt heat mats for snake hatchlings. They do get pretty warm, but if you leave a gap between it and the cage, like with the hot water bottle, it should stop the plastic getting hot. If you keep reptiles and have a thermostat (matstat) brilliant, because you can set it at a low temperature.
I've also heard of people filling old water bottles with warm water, wrapping in in fleece or a blanket and putting it next to the nest.