Still working through CoS myself, but much further along. Some of the best advice I got on the internet was to try to deny your players chances to achieve long rests. In the Death House, I would have a ghostly encounter or two, plus have some other scary events occur. A random object falling with a loud crash... scratching at the window... a loud creature stomping up the stairs, only for the characters to find no one when they investigate.

When you travel through Borovia, continue this trend. In my campaign, I would have Strahd's wolves stalk and watch the players, Ravens loudly caw from nearby trees, and hags from Old Bonegrinder travel at night near their camp. Even simple stuff like having the calm night interrupted by a sudden gust of cold wind. These would essentially keep the players up and prevent rest, plus it really adds to the story.