My triggers

- physical triggers: for many of my characters, these kind of triggers helped while preparing to enter the stage. 
I used to do a lot of push-ups before entering stage if I had to express anger, fear, fatigue or any state that involved an increased heart beat rate. Another thing I did on a few occasions was to inhale and exhale faster than my natural breathe rate
- writing a monologue about previews circumstances from the character's point of view. For example, when preparing Karenin, half of the monologue was the background story, containing facts that were relevant to his temperament. The other half was detailing his feelings just before that scene. I didn't actually read the monologue before entering the stage, because there was no need - it was already in my mind and all I had to do was walk around being nervous and repeating to myself "I'm so angry! I am going insane! How could she do this to me?" etc. 
- connecting with my stage partner - looking in the eyes of my partner and (sometimes) hugging eachother etc. if our relationship was of a romantic nature. Similarly, simulating a physical struggle by clenching fists with a rival character or staring provokingly into eachother's eyes.
- costumes and make-up helped a lot in some situations. Extreme examples are: wearing a tight costume and black wings as Oberon or letting my hair grow and using white spray on it when playing an old character 
- a song that is relevant to the character: Triepleff / Lysander, during the scene
- when playing Lopakhin, an illiterate parvenu who just became the owner of a huge estate, I used to look outside the window and imagine that everything in my sight is mine. There were some very tall trees and watching their movement in the wind, thinking "all this is mine", helped a lot
- I don't know if this is considered an actual trigger, but identifying the space in which I'm acting from the moment I enter the stage also helped in some cases