LAtely, I'm plagued more and more by forgetting the lyrics to songs I thought I knew well at one time. Also, I find that when sitting in a jam situation people keep coming up with songs that I know and have forgotten I know, so I haven't played them in years.
My (far from perfect) solution is that I've recently begun keeping an account book in which I write down the titles of songs know and songs I want to learn/relearn. Then I've set myself a goal of learning a new song each week and playing through the list that I know already (or think I do) at least once during the course of a month.
I'm also finding that I'm using (and even paying attention to) set lists, which I never used to do when playing solo. I don't know if this is a function of aging or using more brain power for other activities or what. But just like my waistline gets out of hand if I don't work at keeping it where I want it, so does my repertoire of songs and tunes.