Remembering by heart - Practice by: David from piano-play-it.com
What I suggest is that you practice memorizing each and every line apart.
Don't try taking it all at once...
Memorize the first four chords, then try to play it by heart.
When you feel you have in, move to the next line and once you have that one two combine both lines and see if you succeed playing both line fluently.
Don't try to play the whole song at one day. Our mind can grasp only a number of new details in one day.
The next day (And not two days later because you'll forget) practice first the first two line you've already practiced and only when you have it fluently move to the next line and again once you have it combine it with the previous two from the day before.