• LOTS of scrambled questions so that students would have a difficult time finding other students in class with the same questions.  I do this for multiple-choice, long answer questions and even short case study questions.  Most online learning platforms allow for question pools, so this is fairly easy to set up.  
• Rewording existing M/C questions from publisher test banks. Since most multiple-choice question banks from the publishers have been compromised, I have reworked these questions and changed the wording so that students can't google the question.  For example, I change people's names if the name isn't pertinent to the question or use different words that mean the same thing (can use a thesaurus if necessary).   
• Less time allotment.  I have cut down on the time limit for exams so students don't have time to look up every single question online or in their notes/textbooks.  I warn students ahead of time that they won't have time to look up every question. 
• For senior courses where multiple choice wouldn't be appropriate, I write my own short case studies and have several in play for each exam.  Some of them have only small differences that would make it obvious to me if they were collaborating with another student with a similar case study.  Others are completely different.  For these exams, I use TurnItIn to check for similarity.
  

These strategies seem to be working, though I know it's impossible to catch everyone.  Looking forward to hearing other strategies!