Someone once told me that you don't really "know" something by just getting it right, it's practising until you never get it wrong. Having said that, it doesn't always work. I have forgotten the "falling of a log" stuff I have been singing for years from time to time and, as someone said above, singing it out the first couple of times is a total different experience. Write it out if you have a visual memory, sing it, keep singing it - it gets there eventually.