One of the big principles asserted in David Allen's Getting Things Done is the importance of having a regular “dump” of what's in our brains into some type of capture system. Some of his process of capturing what's on our minds happens in a somewhat prescribed manner, where we look at what projects are on our plates and what next actions relate to each of them. He also has some great trigger lists, where we can read through a list that is designed to trigger our minds to think about stuff that needs doing. The second method of capture recommended by Allen comes more in the moment, as we remember something that needs to be addressed.
I have a few apps that I use when capturing what's on my mind and getting it recorded somewhere. Each of these apps is located on my home screen, so I have easy access to it.