For anyone finding that they are drifting flat - One trick from choral singing - sing with a smile shape to your mouth to brighten the note and hopefully stop it drifting downwards.

Another trick which may be of limited value here is, when singing a repeating note (ie: the same note more than once in succession), hit each one as if it was a different note - retune it as you go - aim slightly sharper each time to stop it flagging. Mostly a trick used by altos, tenors and basses in choral singing.