When the holidays start rolling in, it's also the time that most bike racers start putting lots of miles under their wheels or hours in the saddle. Everyone has their own methods, but there are a few things that I think are important in order to do it right.
The point, overall, is to build a strong aerobic base that you will work off of for the race season, from spring to the end of summer.
If you do it correctly you will start the season able to build up your top end speed without overtraining. If you do it incorrectly you will either not have the base fitness to build up from (and so your high end won't be as high and/or you can't cope with race pace), or you trained too hard and actually need to rest and recover instead of work on top end speed.
Here's how I think you do it right:
LSD: Long Steady Distance
The key here is steady. You want to always be pedaling and putting a steady effort into the pedals. It should be enough that you can feel it - the point right before you can't comfortably have a conversation. And you have to do it for hours and hours. It'll wear you out after a while. You're teaching your body to have a generally higher level of minimum aerobic capacity.
You have to do LSD rides frequently. 5/Week or more is a good number, but this depends on the length of the rides. If you don't do them frequently enough your body will not make the adaptions it needs to raise your overall base aerobic fitness.
You have to build base over many weeks - at least 6 I believe, but 8-12 is where it should be. It takes a long time to create a big, strong base.
You have to be confident that you are doing the correct thing that will make you go faster later. This confidence will help you avoid going too fast, or getting carried away in faster winter group rides and contesting sprints or pulling the peloton around.
Here are common mistakes (I'm definitely guilty of several of these):
LSD: Long Slow Distance
Some riders will go so slow that the effort doesn't cause any adaption. They go more of a recovery pace than a base pace. It would be like walking to get faster at running.
Some riders will do alot of miles, but won't be able to help themselves and go fast. Some do it all the time. Some do it just at some group rides. This isn't building base, and these racers run the risk of hitting the wall halfway through the season and needing a break.
Infrequent and Inconsistent
Building a good base takes a lot of time - if you can't commit to over 10HRS/Week for 8 weeks, you're better off building up some fitness through other means (more work at threshold, things like that).
If you're doing base right, it'll get pretty boring after a few weeks. If you're doing it on the road, it's best when you can find other guys at your level so you can all ride together at the same pace. If you're doing it on the trainer, hopefully you've got Hulu or Netflix so you'll have something new to watch (my wife's always wondering when we open Netflix "Who was watching The Punisher?").
In the end there are few amateur racers that I think do building base totally correctly. I have a hard time thinking of more than 3. I try my best but think I usually train a little too hard, and this season am committed to trying to truly do some base training - on the trainer in the basement. That way I can really control and track the effort. For all the other adaptions that need to occur, like muscles for stability, I'm continuing commuting on my bike but at a recovery pace.