Question: What should I do if I find myself going off on a tangent when giving a presentation

This is one reason why rehearsing your presentation is so critical.  Practicing increases the likelihood that you will stay on track and hit your key points without getting a case of verbal vomit.  However, if it does happen use this simple strategy:
1.    Stop talking!
2.    Take a deep breath and collect your thoughts.
3.    Then say, “I’m sorry.  I’m getting a bit off track.  Let me rephrase that.”