So, I've recently purchased Curse of Strahd and I'm running it pretty soon; however, with this being my first big campaign (not just a one-shot), I've been pretty nervous about messing up or ruining the experience somehow. I do have experience DMing and being a player, I've played in multiple campaigns, and I've been DMing for a couple months now (with the largest game I've run lasting about 3 sessions).

With that out of the way, does anyone have any advice for making CoS more interesting? Does the campaign ever get boring? Should I purchase expansions? How much should I plan?

If there's anything else you'd like to add, feel free.

My players are going through Curse of Strahd right now.  We are nowhere near finished, but here are some things I did:

1:  Read this guide from Elven Tower, it gives a lot of good ideas:  https://www.elventower.com/curse-of-strahd-guide

2:  Buy the Tarokka Deck:  Tarokka Deck on Amazon

3:  I bought some weathered-looking paper and a wax seal kit to create the handouts for the players (letters, invitation, etc).

There are some other things, but these were the basics.

Hey, I've been running CoS for about the past year or so. I tried my hand at homebrewing a campaign once, but I would say this is my first solid attempt at DMing something long-running. 

In my experience so far, some tips I have are as follows

• While the land of Barovia is vast and you should allow the players to explore as they want, I would try to push them early on to visit the Vistani camp for the card reading session. I did not force my players to do this as I felt I had already railroaded them in the first session a bit much, but I think that giving the players the readings will help to fuel and guide their investigation of the land
• Try to make the most of the gothic nature of the land. When a player casts a spell, have it unexpectedly take on a spookier cosmetic change (covered in the module with examples). Really try to show how downtrodden and depressed the populace is and how dreary the days are. 
• For every major location, try to use one of the optional events mentioned at the end of each section, especially those that feature Strahd. As the module mentions, Strahd is expected to pay the players a visit on various occasions, toying with them and testing them. 
• Try to make Ireena Kolyana an interesting character. Unfortunately, I feel that I have failed in this step. She is a central character to Strahd's goals, so it is important the party keeps her from harm, but I can't help but feel my party sees her as a burden. The book does not formally give her stats, but makes her look like she can hold her own. Try to make her useful in a fight or at the very least a helpful guide to Barovia 
• In my campaign, I used optional rules from the DMG for Sanity(as a 7th stat) and Madness. Outside of the occasional mention in the module, Madness isnt a mechanic used alot. I tried to find areas to implement it as well as replace Wisdom or Charisma saves with Sanity saves instead. This not only fits the theme of CoS better in my opinion, but also makes resisting madness and certain spell effects more difficult (as no class in inherently proficient in Sanity saving throws). I did make a homebrew feat (which is on D&D beyond) which grants proficiency in the save, in case any players wanted to become more robust in their Sanity. 
• If you plan to play on the computer or using a TV display for maps, I recommend buying the digital map set. My group plays on Roll20, and their CoS module is fairly cheap and has everything you might need. 

I think thats all I have to say. Hope this helps