Learning to play by ear is a painstaking, but very rewarding process. I'm not sure if it's tunes or songs you're speaking of, or both, but the main thing is to listen, listen, and listen, and force yourself to rely on your ea. One can lean by taking a tune or song in "rounds," meaning, find a recording of the music, and play the whole number over and over again, each time picking up a note or two. Another way is by breaking up the music into more easily digestible sections, taking it a little at a time. You just might amaze yourself how much your ear can improve when you force that kind of learning on yourself. Ironically, it might help you be a better reader, as well, because after hearing how tunes should sound (again, if you're speaking of tunes) you will be able to interpret the little black dots on the page better than before. Maybe the same goes for singing? I think sheet music definitely has it's place, though, because you can just plain come into contact with more music, and, again, will be able to interpret it better. Hope this helps.