When it comes right down to it, there are just limited outdoor activities that you can do if you live in harsh winter climates. Indoor exercise is boring and stagnant to many people so it becomes looked upon as a chore in comparison to outdoor exercising which makes you feel healthier. There isn't too much you can do to change the fact that you'll have to move your routine inside. But what you can do is make it short. In fact, you can cut you workout time in half just by varying its intensity. Switch back and forth between shorts bursts of high aerobic exercise that really gets you huffing and rest periods where you just calm it down.
Next, pay attention to what you're eating. Not just what you are eating every day but how it looks, smells and tastes in your mouth. If you can enjoy your meals and snacks rather than just wolfing them down, you will feel fuller and more satisfied. Of course, this all goes along with trying to stick to a healthy diet and pattern. A good rule of thumb is to eat three moderate meals each day and two healthy snacks. Stick to fruit, vegetables, lean meat, whole grain and fiber-rich foods. Eat nuts and cheese for snacks instead of sweets or processed foods.
Also, identify patterns. If you've always gained weight in the winter, try to figure out why. If you know why, do something to alter that pattern. Many people eat out of boredom. Try a new hobby or take up a class.
Big holiday times occur during the winter and this might be the biggest culprit of all when it comes to winter weight gain. If you're cooking for the holidays, make sure you keep raw vegetables nearby to snack on and also serve healthy dishes as alternatives. If you're a guest at many parties, try to go for the healthy choices first.
Buddying up with a partner or joining some sort of support group can help too. It's much easier to stick to your initiatives when you're held responsible by someone else who is going for the same goals.
Finally, remember that winter weight gain is perfectly normal. While you don't want to be packing on the pounds, don't get worked up over a couple of extra ones. These can be lost almost as easily as they were taken on.