If you learn a song but don’t play it for a while, you will gradually forget it, but you’ll easily remember f you’ve learnt from memory in the first place.  It requires a little work, but not too much.

The principle is simple but obvious: the skill you want to develop is that of remembering, so that is what you have to practise.  Not repeating - remembering.

1. Play the song through a few times with the chart, paying attention to the harmony – the movement of the chords.  Where are the I, IV and V chords? (See Nashville Numbering System)
2. Put the chart down and try to play it from memory.  You’ll get lost, just laugh at yourself! – it’s normal :  )  What will happen is that you’ll remember some of it.  Perhaps the beginning and the end. 
3. Look at the chart to see where you went wrong and try again.  The second time, you’ll be surprised at how much more will sink in.  Keep going and you’ll have it. 
4. If you get to a part that you can’t remember, just work on that section for a time until you have it, then go back and play the whole thing.

One thing: there is a danger that you will end up learning only where your fingers go and not the harmony.  I would do two more things, once you have learnt the song:

1. Pay attention to the relationship between the chords you’re playing.  Does a V7 go to the I chord at the end of a section?    ( Nashville Numbering System)
   
2. Play from parts other than the beginning.  You can learn enough to play the song from start to finish, but you want to know the song properly.  If you’re in a band or duo and the singer makes a mistake, you’ll need to follow them.  Once you’re a musician, you’ll see this happening in performances.  A good band will cover it up.