Making sure you're relaxed before hitting the hay guarantees that there'll be less tossing and turning and more dreaming. Winding down with a warm bath, just before bed, is a sure fire way to gently make you sleepy and relaxed. Warm milky drinks have the same effect if your stuck for time. These methods keep you warm, by heating your core, rather than just the area around your skin and body.
Gentle exercise encourages good blood circulation, which helps to keep your body warm and regulates your temperature. However, don't over do the exercise before bed, as you may feel too invigorated to sleep in the first place.
Keeping the bedroom itself at a consistent warm temperature will also help keep away the night time chills. Ideally the room should be warm enough so as not to disturb your sleep. Too hot or a terrible draft, will cause you to toss and turn, breaking up your regular sleeping pattern.
As mentioned before, a thicker duvet is the most common choice amongst the general public, as they prepare for the colder nights. However, other parts of the bed, such as the mattress, can also be adapted to help keep the temperature up. A soft, thicker mattress is a much better insulator than a thin, flat one; using a fleecy underblanket to retain the heat is also advised.
If a completely different duvet is out of the question, then layers on blankets and sheets is just as effective. Layers trap the warm air and they are also easily removed if you become too hot during the night.Other little ideas include cuddling a hot water bottle during the night (making it sure it has a cover over it, to avoid scalding) and investing in an electric blanket. If used underneath, the heated blanket will warm up the bed before you retire for the night, but used over the bed it will maintain a constant temperature throughout the night.