There are tricks that work - but no matter how well you know a song there's going to come a time when it just goes walkabout. It's a bit like when a cartoon character runs off a cliff - there they are running along in mid-air, and then they look down, and fall like a stone.

Having the safety net of the words is fair practice I think. And it can make it less likely that you forget the words, because that worry is removed. (Reading the words as you sing them, that's different, and it hardly ever seems to work well. It's like the difference between telling a joke and reading it off the page.)

Mind, forgetting does have its creative part to play. When you forget a line or part of a line, and improvise to fill it in, that's very much part of the process by which songs change, and often improve along the way. And it can happen unconsciously - I know that when I check back on a song I've written a long time ago I always find it's got lines different from the way I have come to sing it - and the change is always for the better.