Google has added a really nice ‘Tasks’ feature to gmail which you can use to manage your things-to-do lists right there in your gmail account. It is still an experimental feature so you will need to enable it by going through the Settings–Lab tab.
You can create tasks by entering a single line description or from within an email. It will even link that task to the email using email subject as the text for the task (you can change it though)
If you are still keeping everything in your head, take a break and off-load your thinking to gmail tasks list.