Write It Down
Sometimes I'll hear a phrase or a word and write it down in my little black notebook (a writer's best mate), then come back to it and work a plot around it.
Writing down my ideas as soon as I have them has made a huge change in my life. You would think that since remembering our good ideas would certainly give us an advantage biologically, that our brains would have adapted by now to be good at remembering them. Unfortunately, this is not the case.
I've read that the human mind is able to store between seven and nine things in our short term memory. This means that your amazing idea has an incredibly short time until it is crowded out by the mundane thoughts of "did I water the plants?" and "Where should I go for dinner?" The science of this is surely more complex than new ideas pushing the old ones out, but if you care about remembering your ideas, you should treat it this simply.
I imagine that my ideas have a two minute lifespan. If I have an idea, and think to myself "I should write that down later to remember it," if I don't write it down within two minutes it will be completely forgotten. Sadly, sometimes I've forgotten ideas even within two minutes of having them.
In order to have a place to write down my ideas, I keep lists for different topics. I have a list of blog post ideas, a list of ideas for each of the organizations I'm involved with, a list of ideas for businesses, a list of short story ideas, a list of books and movies to see, a list of random things I want to learn, and lots of other lists.
These lists are kept as simple text files. I use an app called PlainText on my iphone that links to my dropbox folder. This means that wherever I am, the lists are always with me. It takes me 15 seconds after having an idea to open the right list and type the idea in. Additionally, the lists are automatically backed up and synced on all of my computers, and are just text files, which means if PlainText stops working, it wouldn't be a big deal.
If you're not doing something like this, try it out and let me know how it goes.