Before a class, rehearsal or performance, it's good to take a few moments to prepare. Your aim is to stretch, release any tension, and focus your attention outward to anything the other actors or the scene may throw at you. The last thing you want is to be looking inward and judging your own performance. There are enough critics in the world already.
Here's an exercise you can do to centre yourself and release tension. Remember not to hold your breath.
1 Stand with your feet a shoulder width apart.
2 Stretch up with your left arm on to your toes as if reaching to find a key on a high shelf.
3 After 10 seconds lower your heels to the floor but keep your hand searching for the key.
4 After 10 seconds drop your arm and follow it down to the floor with your upper body hanging from your hips.
5 Rest for a moment then reach up with your right arm and repeat.
6 Drop again and hang there. Your knees should be slightly bent and pointing in the same direction as your toes.
7 Sway slightly keeping your spine and head free.
8 On an in-breath, slowly build up through the spine, vertebra by vertebra, until your head comes to rest floating on the top.
9 Your eyes should be open and engaged with what's in front of you.
10 You should have the feeling of being centred and released.
When you are about to rehearse or play a scene, here are a few ideas as to how to look at the text but keep your attention outwards.
1 Try to go through your lines while doing something else (I juggle while saying my lines). Maybe get into costume or apply your makeup. The split focus, on the activity as well as remembering your lines, is a good simulation of being on stage; your reflexes are ready and reactive to what's physically around you while your brain takes care of the logic behind the lines.
2 Go through the logic of the argument/dialogue step by step without the lines. Find the flow of the argument.
3 Find the cue word or phrase in the other character's dialogue that makes you want to say your line. This helps you to focus on the other actors and exactly what they say rather than just listening out for your cue.
It is also recommended that you do a full vocal warm-up before you hit the stage. If you engage your voice you also engage your ability to react spontaneously and express your reaction. That's what the audience are playing for even if they don't know it.