Hydrating for Winter Training
For obvious reasons hydration is a huge topic in the summer when big increases in sweat rates during training (and even during day to day activities if it gets hot enough) can make it tough to keep fluid levels balanced. However due to a combination of increased amounts of indoor training, wearing more clothing layers and exercising in colder, drier air the importance of good hydration practices in the winter months should definitely not be disregarded.
The first thing worth noting if you are training indoors in the winter, often doing shorter more intensive sessions, then showing up to them adequately hydrated is a sound plan if want to perform at your best and recover quickly afterwards. Sweat losses during even a 45 minute indoor set can be pretty massive if you are working hard and it is no good just aiming to drink lots when you're training on the turbo or treadmill. That is the classic 'shut the door when the horse has bolted' approach. Instead work at your hydration in the last few hours prior to your workout by sipping 500 - 750ml (roughly 16-25oz) water, or an electrolyte drink so that your body has time to process and absorb what it needs and eliminate any excess. Any fluid you take in immediately before the session is likely to simply sit in your stomach and do you very little good, especially if you're doing something very hard, like interval work where most of the blood flow is directly away from your gut and to the working muscles, further reducing the chance of it being absorbed quickly.
For the most part when considering shorter indoor training sessions you should not need to drink much at all during the workout, assuming you started well hydrated in the first place. This is true for anything up to 60 minutes, maybe more in some cases. It's certainly a good idea to have a bottle or two available, and to drink to the dictates of thirst, but your body is going to have a hard time processing lots of fluid when you are working hard anyway.
If you're going outside doing longer sessions in cold, dry air you might find that you may benefit from drinking a slightly stronger electrolyte drink than normal as there is a tendency for your body to want to pee more in the cold (a well documented but incompletely understood phenomenon called Cold Diuresis). Adding more electrolytes (sodium namely) to your water encourages your body to hold onto fluids more effectively in order to keep the blood sodium balance in check, and therefore you will theoretically lose less water as a result. You may also sweat quite a lot even in colder temperatures if you wear a lot of extra clothing layers.
Recovery from any training session will be improved if you can restore fluid balance relatively rapidly and once again adding some sodium to your post session drinks can help the body to replace what was lost in sweat and increase the percentage of water that is actually absorbed. One thing to remember is that your body can only take in a certain amount of fluid, usually somewhere between about 400ml and 1000ml per hour (approximately 15 to 36oz) so drinking any more than that is probably futile and you therefore need to pace your rehydration out over a few hours post session, using thirst and urine colour/volume as the main indicators of when you are back to where you should be. Ideally your pee should be a relatively pale, straw like colour when you are optimally hydrated not dark (indicating possible dehydration) or totally clear (possibly indicating over-hydration).
Drinking Too Much
It is worth mentioning at this point that hydration, in the past at least, was often portrayed as a 'more is better' topic when it is actually it is actually a balancing act. The body has fantastic self regulatory mechanisms in place to keep your fluid and electrolyte levels in homeostasis; if you give it roughly the right amount of each, with 'roughly' being the key word. Where it can go wrong is when people over ride natural instincts and become convinced of the need to just drink lots and lots regardless, because dehydration is the perceived consequence of not doing so. At first if you drink too much fluid your body will do a good job of getting rid of the excess by simply making you pee more, however keep doing so and you can begin to dilute the level of electrolytes in your blood as well as put on unnecessary water weight, neither of which is good for your health or performance. That is why learning to listen to your body and to read the early signs of thirst is actually the best way to keep you fluid levels balanced.
Most importantly, remember not to neglect hydration in the winter, especially if large amounts of your training is done indoors or in cold, dry conditions where fluid loss can be surprisingly high. Linked to that is to do your best to turn up to each training session as well hydrated as you can be. The best time to think about drinking is not right before you get started, so make sure you have suitable drinks to hand throughout the day and that you top up regularly if you are training in the evening, or that you go to bed properly hydrated if training first thing in the morning. Do listen to your body though and keep an eye on urine colour in order to monitor your hydration status on a day to day basis. Orange or dark pee in small quantities means you probably need to increase fluid intake, but if it's completely clear and you are peeing all the time you are perhaps drinking too much and that is not a good thing either. Pale straw coloured urine in reasonable quantities is a good sign you are getting it about right.
When training drink largely to thirst rather than setting arbitrary targets of how much you think you 'should' be consuming. Just make sure you have plenty of fluids available so you don't run out. Finally, add electrolytes (mainly sodium) to drinks if sessions are particularly long, arduous or if you have an especially high sweat rate. Remember that drinking too much can be as bad, if not worse, than not drinking enough, for your performance and health.