The way I tend to work, in anything creative, is that I'll be working on one project and thinking up ideas for the next. If I don't get tired of it I'll know it's a keeper
Finally, as for forgetting ideas I never really worry. I tend to believe that the most interesting elements I'll still remember, and the only things I'll forget will be the unnecessary or clumsy bits.
This part is really spot on. No matter how much you love it, every project eventually gets out of the Honeymoon phase, and you start daydreaming about what you'll do when you finish it. It's really good to capture those, because (at least for me anyway) no matter how much I think I'll TOTALLY REMEMBER all these great ideas I'm having, the truth is, they are gone within a week. Month tops.
Also, it's really really gratifying to be like "I wonder what I should do next" and then open a notebook FULL of ideas, and spend and afternoon going "oh yeah, that was a really cool idea. I wonder if that could still work...?"