You have to replicate race efforts in training.
Cyclo-cross races are short and sharp – normally about an hour long – so keep your training short, sharp and specific. Do lots of accelerations and top-end work in order to replicate what you will feel in races.
Cross races are really hard even though they’re only short. In training you’ve got to go hard but also remember to train your technique along with your fitness. There’s free time to be had if you’re good technically.
I know a lot of people use the turbo and rollers to replicate race efforts – and particularly those who have a day job and so only have the evening to train – but I prefer to actually get out on the road to do my intervals. You get out of the saddle a lot more on the road and it just keeps it more realistic to the gear you might use when racing.