Les:
Can I just state the obvious and say that the longer you work on a song before you sing it publicly, the more chance you have of making a good job of it, and the more chance you have of keeping it in your repertoire. I wish I had a £ for every time I dropped a song because I made a hames of it the first time out.
MacColl reckoned he never sang a new song in public until he had worked on it for three months - he was prone to exaggeration, but I know for a fact that he put a great deal of time and effort into learning.
The only times I have ever seen him sing anything 'hot off the presses' was when the Singers Club moved to new premises; he wrote a song entitled 'The Song of the Travels' after its first move and added a verse and sang it on the opening night of the move each time they moved .   
There are other 'tricks' to making a song your own forever - Sam and Tish's seemed a good overview.
Jim Carroll