The book On Combat by Dave Grossman discusses some of these things. I haven't read it yet, but have read reviews and listened to a podcast by two people I consider highly evidence-based and reputable (here). In particular, the book discusses a method of physiologically lowering your heart rate he calls "Combat Breathing". This entails 4 phases, each for the durations of a count of 4 (no unit specified, I do approx 4 seconds):

1. Breathe in

2. Hold in

3. Breathe out

4. Hold out

It sounds very simple, but I have heard multiple recommendations of it from both the armed-forces and medical worlds. I can also add a data point confirming it works well for me (mostly only for reducing heart rate to below 100, not all the way down to resting rate).