Easier to demonstrate face-to-face than to describe; but my old choir director used to tell anyone a little flat to "put a smile in your throat."

Most people tend to slightly raise the pitch when they smile over what they get with a "flacid face." It isn't exactly the smile that does it, but a reflex that raises the soft palate (the flabby part of the roof of your mouth) and changes the resonance of the vocal tract slightly upward.

Once you get the feel of it, you can "raise the roof" without the outward smile.

There is a tendency to pitch your voice by "muscle memory" in which you adjust the vocal chords by the memory of how much muscle it takes to get a given note, instead of by listening to what you're singing, and the tendency is to drift slightly flat when you rely too much on that method. If you can learn to keep the soft palate "up" the tendency to go flat is reduced.

It doesn't avoid the need to listen to what you're singing, but adds a little lift. It's most effective if you can learn to sing with a "full throat" that lets the sound echo clear down into your belly; but for "pop songs" that can tend to make one "go operatic" which may not be the tonality you want. For operetta music there's probably little danger of overdoing it though.

Some people find it easier to sing in tune if they have an accomplice accompanist sounding the harmony part for the song. It seems easier to "resonate" (harmonize) in tune with another part of the chord than to match an identical pitch of someone singing (or playing) the same notes as what you're singing - if you listen for the resonance.

People seriously trying to learn vocal arts spend a lot of time singing scales and "open chord" exercises, often with a piano or other instrument to check their pitch, but the practice does little good unless you're very conscious of being accurately pitched on all the notes, by comparison to an accurate instrument.

John