Over 40 years ago, I attended a singers' workshop run by Ewan MacColl and Peggy Seeger. I remember an exercise in which we were asked to produce vowel sounds (aah!, eee!, ooo!, etc. - sounds really weird, I know!). The idea was to produce the sound from the diaphragm and out through the mouth as 'cleanly' as possible and at moderate volume (can't remember the exact terminology after all these years). I found this simple exercise to be a complete revelation! I realised that, up until that point, I had been using only a fraction of my 'singing potential' (the image that came to me was that I had been singing in a 'cupboard' but actually had a fairly substantial 'church hall' to play with). I've been singing, unaccompanied, in public, ever since. I'll never be the world's greatest singer but I can still stand up in front of an audience with reasonable confidence.

The other tip is to think about your breathing and try to take in the right amount of air at the right point in the song, e.g. ensure you have enough air prior to a high note, or a decoration, or before a passage where taking in a breath would be inappropriate for the song's rhythm. Aim to work out the optimum breathing pattern whilst practicing.