Many sleeping bags can lose their performance and heat retention from washing so the solution is to use a sleeping bag liner like the ones below. The additional advantage is that a liner can add 1-4 degrees to the comfort rating of your bag so it's great if you're passing through a range of climates on your travels.
Another great use for liners and sheets is as a sleeping bag or cover when sleeping in warm and tropical climates. They help protect you from mosquitoes and other biting insects and are useful if the accommodation's sheets don't look the best! Most also have an internal security pocket so you can keep your valuables close at hand as you sleep.