Just implying that Strahd is near and watching is often quite enough. I'm still preparing that adventure but back in 1ed (AD&D), I used him a lot. Players knew he was out of their league when the count wasn't turned by the cleric, took the holy symbol the cleric was holding with a yawn, crushed it in his hand and said:"How rude, I thought I had gentlemen in front of me. Be happy dear friends that I'm not so easily slighted."
That scene set the tone for the whole adventure. Strahd was there when they were having a tough night. If it was too easy, a fireball, an ice storm or simply a few more wolves to toughen an encounter were his moves. The players were dreading his presence and they were always watching their back. One player was even blaming Strahd for bad die rolls! All random encounters were, of course, Strahd's fault.
Make Strahd appear once in a while in the distance when the players are fighting. Let him congratulate the players on their success. If they stay polite and act like gentlemen toward him; Stradh won't be an ass. If they sligth him, however small, he will show them proper respect.
1) Make him stay at extreme range for fire ball. If a player tries to attack him. Throw that fireball at all the players save the one that attacked. Your other players will go mad. I assure you.
2) If a random encounter goes very bad. Have Strahd recall the monster to him. "Dear friends, I really thought you could do better than that. How disapointing..." Strahd goes away with a smile.
3) Do not limit Strahd to the small spell list he has in the adventure. Have him stand in the sunlight! He'll use an illusion while he's inside a house. He will call the players, bathing in the sun. "Come on my friends! What are you waiting for? It's quite a nice day don't you think?" And make him transform into a bat and fly into the distance. Your players will start to fear that vampire.
4) Strahd has a nightmare. Make him do a flyby and throw an Ice Storm on the players (or whatever works for you) just prior to an encounter (no time to rest). Starting an adventuring day with less resource is always a strong incentive for prudence.
5) When the players are taking a short rest (or before a long rest); have Strahd attack them with a few zombies, vampire spawns, skeletons and wolves. Strahd stays in the back. Smiling with an evil grin. "I do not feel like resting, neither should you. You will have to earn that right my dear friends. (insert discrete evil laugh here)
6) Add monsters if Strahd needs them to make a strong impression. They have killed the gargoyles? Have them right back at their post the following day. Or have Strahd come to attack the players with double the number of gargoyles they defeated (make it out of the deadly range) and have Strahd say:"Now my pets, these are the ones that killed your brothers. You wanted revenge? Here you go." Make him turn toward the players "I am so very sorry for this show of violence and the inconvenience it might cause you. But you know how it is. I have to prove to my servants that I care for them." (Insert a good old evil laugh in here)
These were but a few of what I used back then. They are still very usable today. When my group finished my Desert of Desolation campaing, They will go on to CoS with new characters. I hope I have been useful.