To follow my examples, it helps to know that I use the iPhone app Captio (which I recommend) to send myself emails from my phone. It will send every email you create to the same address that you specify in the settings. This means that to send yourself a simple reminder note, open the app, type just your text (for example: "wash car") and press send. It's handy if you email yourself a lot with reminders and notes. It also allows you to attach a photo.
In my case, the complete message to myself would be:
xdo wash car
"xdo" is an xword I use to tell my email account to forward the item to my Remember the Milk (also recommended and free) account as a To Do item. In Gmail, this can be accomplished by creating a filter.
Now, why an xword and not a normal word or even a word combination like "todo?" Because that might appear in normal text, say, something someone else might send you. If it does, the filter will act upon that email and you'll need to deal with it each time. An xword that you create should not be a real word. This way, the chances of it appearing in text where you did not intend it to be are slim.
Why not a hashtag or symbol of some sort? I want my solution to be "application-agnostic" meaning I don't want it to work in some places and not others. Software is inconsistent at dealing with and parsing words containing symbols, so to keep it simple, I want to use only alphanumerics.
Another thing I like to track is when I have finished a book. That message will look like this:
xred Peopleware: Productive Projects and Teams by Tom DeMarco and Timothy Lister
This triggers a separate filter which tags the email and archives it after forwarding it to my Evernote account.
xwords are also a great way to "tag" something that doesn't allow for tags. The "tags" can be used anywhere. You could rename a series of photos to end with "_xhivn" so that "dave_hiking.jpg" becomes "dave_hiking_xhivn.jpg" so that you can search for files containing "_xhivn" on your computer to find all of those files which relate to your Hawaii vacation (photos, maps, receipts, etc.)
You can also use multiple xwords, just like tags:
xdo xhivn get plane tickets
As a final thought, it's best not to tell people your actual xwords and the email account in which you use them, for obvious reasons. ;)