When making a good plot, I often find it difficult to keep it coherent because as I grow older, my writing becomes better (as it naturally would, I hope!) But this means if I'm working on a novel or something very long, which I am at the moment, then new ideas for the plot keep springing into mind to make it better than it currently is. This means the plot keeps taking a lot of different twists and changes in direction, and is unstable.
What advice would you give?