Before a performance or practice, go over this checklist in your head:
1. Visualize the mouth shape of each target vowel
2. Sustain target vowels till the end of the word.
3. Be super sensitive to the pitch you are going to be singing and listen carefully to the parts around you.
4. If you have the third of a chord, back off. You don't have to be as loud in relation to the other parts.
5. If you have the root or fifth of a chord, come out more, but don't oversing, that tends to distort the pitch.
6. Take a big belly breath every time you breathe when singing. Open the throat so there is no breath sound when you take in the breath.
7. Sing legato, no breaks in the sound unless they are planned.
8. Drink some water prior to singing.
9. Watch the director at all times.