The weather outside is colder, the leaves are off the trees and the amount of races available has drastically dropped off.
Don't get the racing blues—this is arguably the most important part of your racing year/macrocycle. The macrocycle, which is 12 months for most athletes, starts with the "base" phase for the first "mesocycle."
The base phase, usually starting in the winter, offers great opportunities when the weather turns cold. During the base phase, the main goals I have for my athletes are to work on form in all three disciplines (swim, bike, run), build strength and create a large aerobic engine (cardio system). When these three things are accomplished, it allows the athlete to go into the next mesocycle stronger, with less risk of injury and a large base to build speed off of.
This training allows us to work on great technique.
In the pool, along with the usual interval training, spend time on drills. I highly recommend the armpit and fist drill as well as swimming with a band around the ankles to improve body position. Also, ending a swim workout with a 500 yard pull set will not only build sport specific strength, but also aerobic base.
When the weather doesn't allow us to get out on the bike, a trainer is not only affordable, but very beneficial. It will provide a consistent workout without stop lights, downhills, etc.
To build the aerobic base, turn on a movie or watch a football game while you spin for a couple hours. During this time, work on form (keeping heels down) and a high cadence (problem for a lot of runners). Not only will this help burn off the Christmas cookies, but will build a great endurance base.
The dreaded treadmill also has benefits for us when you're stuck training indoors. When over-striding on the treadmill, it will let you know (jamming the foot into the belt, the motor will bog, giving feedback of improper form). Also, it provides a cushioned surface to prevent injury and helps with pacing.
To keep the treadmill exciting, work in short speed intervals to work on turn over. Get outside even if the sidewalks are icy because most of the time, trails are not. Trail running not only is great aerobically on the uphills, but helps increase leg turnover on the downhills. It also helps build sport-specific strength with the hills and the uneven footing strengthens hips and ankles.