Lots of great advice already posted. See if your local hiking club has a winter hiking seminar. I also want to emphasize how important food and hot beverages are. In addition, at the end are a few cold-weather tricks that really help me to stay warm.
FOOD: You should try to eat almost twice as much as you normally would, since your body will need a lot of fuel just to keep warm. Snacking as you hike is better than taking breaks to eat. Carry snacks cut up into small pieces in your pockets (your body heat will help to keep them from freezing). Pieces of cheese, nuts, pepperoni, chewy dried fruit (apricots), even pizza are great cold-weather snacks. Most energy bars get very hard to eat. I like Odwalla bars since they stay fairly soft, even in very cold weather. Keep meals simple - make quick one-pot meals that require only boiled water so that you're not waiting around getting cold before dinner is ready. I like cous cous with dried vegetables and cheese, Minute Rice with pepperoni and dried vegetables or Mac & Cheese (always good!). Add olive oil or butter to everthing for more calories. Have a snack (such as chocolate or candy bar) right before you go to bed so that your body has enough calories to burn in order to keep you warm all night.
BEVERAGES: Carry water bottles upside down in bottle insulators instead of Camelbaks or other bladders. Even insulated bladders will usually freeze below 20 degrees (I know this from experience). Adding Gatorade usually will help to keep the water from freezing. Carry a 16 oz. thermos filled with hot soup or tea to keep you warm during lunch or breaks. I fill mine at breakfast and have half with lunch, then finish the rest when I get to camp, since it's still piping hot even at 4pm. Make a hot beverage or broth as soon as you set up camp, since your body will cool off rapidly after you stop hiking.
TRICKS/HINTS:
Hot Water Bottle - Just before going to bed, fill a Nalgene bottle with boiling water and make sure the lid is on VERY TIGHT. Bring the bottle to bed with you and place it between your legs or wrap your arms around it. This hot water bottle will keep you very warm for hours.
Down Jacket and Booties - Invest in a good lightweight, compressible down jacket with a hood for when you get to camp. I like the MontBell Alpine Light Parka. Do NOT hike in the jacket, since it will get wet if you sweat and the insulation does not work if wet. At night, put the jacket in your sleeping bag with your feet in the hood to keep your lower half warm. Also, take off your boots as soon as finish hiking and put on a pair of down booties and dry socks. This will keep your feet dry and very warm. Booties with rubber soles that you can walk in are great for late night "bathroom breaks."
Boots - As someone stated above, take out your boot insoles out and put them in the bottom of your sleeping bag to keep them from freezing. In addition, put your boots in a stuff sack (your sleeping bag sack will work) and keep them in your tent so they will be less likely to freeze. Nothing harder in the morning than putting warm feet into frozen boots.
Socks - Try an insulated sock liner, such as a merino wool liner or wool/silk. Make sure that you don't put on too many heavy socks that will decrease your blood circulation. Blood needs to be able to flow to keep your feet warm.
Clothes - Always keep your set of sleeping/camp clothes protected in a truly waterproof compression sack. You must have warm, dry clothes ready to change into as soon as you stop hiking. For hiking, consider wearing base layers of merino wool, which is very soft, will keep you warm without a lot of weight and move sweat away from your body. When hiking in 25 - 40 degree temps, I wear a light wool (or fleece) hat and gloves, a lightweight wool tank top, a lightweight insulated zip-neck top and a fleece vest on top. That's it. I find fleece jackets cause too much sweat. If it's really windy, I'll put my rain jacket over the rest with all the vents open. For bottoms, I wear lightweight wool leggings under my rain pants or a lightweight pair of soft shell pants.
Extra garbage bags. These will help to keep wet items in your pack from getting other items wet. If there is frost on my tent in the morning, putting it in a garbage bag will contain any moisture as the frost melts.
Camp Activities - Gathering (dead and down) wood for a fire if there is a fire ring will help to warm you up. Playing charades after dinner is a fun thing to do with a group before bed. In the morning, pack up your sleeping bag and tent before making breakfast and you'll warm up quickly from all the movement and effort to stuff everything back in the stuff bags.
Winter is an amazing time to be in the woods if you're well prepared. Less crowds, open views, no bugs. Enjoy!