Lots of helpful comments here, and many have said things which I would have.
Whenever i have an upcoming opportunity to sing in public, i always take it
very seriously and practice my repertoire beforehand. The songs i sing are ones that I love and want everyone to hear, so I make sure that I do them justice to the best of my ability. Otherwise, why bother? Singing in a casual setting means relaxing standards a bit, but you still want to do the song justice.
My biggest faults are not relaxing my body/breathing properly. It's
something i have to consciously work on. Experiences singing in a musical and with a short-term choir certainly provided me with valuable tools to improve my singing - even if I did find some of the exercises a bit daft.