How are you keeping track of features requirements/details when you have those moments of 'oh yeh I cant forgot to add x'?

So for example you're working through your wireframe and then you realize that you'll need x piece of functionality as well. How are you keeping track of these sorts of things?

At the moment I've got a project setup in Things where I just add new tasks every time something pop ups which I then process later.

This process works ok I guess, but if there's a better/smarter way to manage this can someone let me know.