My advice is to follow our help articles to the letter. The vast majority of the support cases we get about sync problems are because people skimmed the directions, or even worse didn't read them and thought they could figure it out on their own. Follow each step word by word, and double-check everything.
Additionally, whichever user you create in Salesforce to be the sync user, make sure it has a valid email address which your marketers are remembering to check, or better yet it's an alias that sends to multiple people. If it's not a real address or people aren't checking it, you'll miss the password expiration notifications from Salesforce and then your sync will go down when the user's password expires.
Check out my Guide too