Some songs it seems that I knew forever. Others including some that I have written I still get mixed up in. I usually try to sing from memory rather than from a sheet and my memory is good for the old stuff even when it seems that I can't remember my name. I learn new stuff by singing into the computer using "audacity" and repeating it over until it sounds "right to my ear" and I am confident that I have it down pat before I let 'er fly in public. Repeat and repeat.