When I was a kid, I wrote a few main stories. However, I also had tons of ideas, and I wrote all the time on those, too.
I had a 3×5 card I used to carry around to keep track of all the titles – there were about 15 stories I constantly bounced between.
I read a lot of books, watched movies and TV, and played video games. I was inspired by many different things. So, I tried to keep track of the stories with multi-subject notebooks. Every day, if I had an idea for one of the stories, or if I was bored of the one I was working on, I’d just switch to the appropriate notebook and write a bit on a different one.
I don’t do that so much anymore; I don’t write on so many different stories at the same time. Instead, I find myself developing many areas at the same time for a single story – locations, characters, backgrounds, world events, etc. For a while, I just used folders and Word documents as my multi-subject notebook dividers. Eventually I switched to a wiki. It gave me the ability to separate topics out but easily link between them, and I’m loving that.