Many artists keep a journal, not only as a record of the past but also to help themselves progress. Photographers are no different. Often, a concept or idea will spring to mind but, for whatever reason (time pressure, lighting conditions, state of mind), needs to go on the back burner. A journal lets you keep track of your ideas and thought processes to revisit and build upon later. For a bit more on the subject, check out this video on The Mindful Eye website (alternate link here if you don't want to register; but you should - it's free and there's bucket loads of great video content).
Anyway, a smartphone (like the iPhone) makes journaling simple. It's a camera, note pad and sound recorder in a compact package that you take with you everywhere. There are lots of different smartphones out there, with hundreds of thousands of software applications (or 'apps'). My own phone runs the Android operating system (open source and based on linux). The app I've been using to journal with is called Catch Notes, available for free on the Android Market.
How does it work? Let's say you see something you want to record in your journal. Start up Catch Notes and you'll get the following intro screen:

Catch Notes Startup Screen
To create a new entry, hit the plus symbol (top right). You'll enter this template:

Template for your journal entry
Touch the paperclip and you'll be given the option to take a picture or attach one you've already taken. You can also dictate a note or attach a recording of the sounds around you (just don't expect high fidelity).

Attachment Options
In this example, I take a picture...

Photo for journal entry
...and a new icon appears in the lower right of the note screen, telling me the picture has been attached.

Note with photo attached
At this point, type whatever you feel like in the text field. I prefix my journal entries with the tag #photoj so I can easily extract them from my other notes. Any tags you've used are listed along the bottom of the screen.

Completed journal entry
Hit 'Save' (top right) and the journal entry is complete. The next time you open Catch, you'll see a list of all your notes:

List of notes
You can use the search function (magnifying glass) to pull out your photo journal entries (using the #photoj tag) or be more specific, such as searching for all entries mentioning 'HDR'. Tap the search result you want and the phone brings up the entire entry to review or edit. (It often reads better with the phone in portrait mode):

Your finished journal entry
Catch will also geotag your notes. You can later call up a map of where the note was made, or search for other notes made in the same area.
You can create a Catch.com account and synchronize with your phone. This gives you the security of a back up and the ability to view and manipulate your notes online using your home computer and web browser. If you sign up for an account, the Catch.com people say they'll keep things private. You'll have to decide whether to trust them with your personal thoughts / million dollar ideas).
So, no more excuses. Get your journal on!