Most of the time I get the tune and then try and find a set of chords that will go with it, to help me remember the tune.

Sometimes I'll make up a tune with the guitar in my hands - but I'll normally be thinking melodically rather than harmonically. I'll have a note, and a chord will come along with it. Maybe I'll experiment with the different chords that would include that note.

And when I've got it more or less right, I'll go around singing it without an instrument. Probably when I get back to playing with it I'll find that the tune now needs slightly different chords.

And the other thing is to try playing it on a different type of instrument, like a concertina, or a bouzouki.

But I find it very hard to avoid ironing out the complexities of a song when I'm playing an accompaniment to it, especially with other people playing along.