Synchronise beginnings and endings of words with the lead vocalist.
Pick out your note from a chord either from the guitar or keyboard, it will be true to pitch.
Observe pronunciation habits of the other singers to create a smoother blend
Use good posture, as this will help you control lots of things about the note – the tone, using controlled breathing, the pitch, and a better sound in the mic, as you have a directed sound.
If you need to sing a specific note within the harmony, sing throughout the bars leading up to that section even if it does not go over the preceeding chords.
Pick a ‘reminder note’. Is there a word in the lead melody that is the same as the note you will be singing?
In your head, sing along with the lead vocalist all the time so that you can match the dynamic impact of the lead vocal when the backing harmony comes in. This helps you to tune in to the intensity of emotion the lead vocalist has lent to their vocal.
Always remember, you are a supporting instrument. Sing the BV’s for the sake of making the song better. Play with timbre and tone to create the underpinning, without trampling all over the lead vocal.