It's practice, gillymor, when we were young most of our time was spent learning new things, and we practised, practised, practised all the time.
Get yourself over to Duolingo and decide on a language to learn - better still, pick two. Duolingo is basically a simple computer game that drills you while teaching you new words and phrases. Very addictive - you see people with 10 or 20 languages! And you find the language slipping seamlessly into your mind, and with it your ability to learn reappearing.

The other thing about learning by heart is to do it while you're active. At the moment I'm learning the Internationale in French, on my almost-daily cycles downriver to the sea and back. The fact that you're moving rhythmically, and the fact that you're in fresh air, and the sight of flowing water all contribute to learning by heart.