Practice - necessary, but distasteful! I try to practice scales to limber up my stiff fingers, but learning a song is a whole new world. I play all my songs from memory with the sheet music there for a quick backup peek if I need it. I have mostly played by ear from my earlier days when I was learning guitar and couldn't read music. I would listen to a song on tape for a few bars then practice until I had it down pat, then move on to the next measure. It was a lot of work but the only way I could learn a song, and when I switched to console organ, things just fell into place and playing by ear worked well. I forced myself to learn basic notation so I could read simple music scores from fake books, etc., and that's how I do it today. Learn the song from the sheet music, play it over and over until it becomes second nature, make changes you like to hear (arranging), then record it. Sometimes I will listen to an earlier song I recorded and wonder how I managed to do that - forgetting the hours I put into the preparation. Playing by ear is something you just acquire after years of practice, plus it helps to have an ear for music.