This tip is easy, but extremely useful for users wanting to get the most out of their iOS device. Interestingly, there are tens, if not hundreds, of calendar, to-do, and organization Apps for your iPhone, iPad, and iPod Touch that you can use to remember things.
But the best way may be a functionality that iOS offers already. The screen most people see the most is the lock screen. It stands to reason then that the lock screen would be the perfect place for a constant reminder.
There are any number of ways to accomplish this. One, provided by a Mac OS X Hints user is to simply open the included Notes App and type out the message you need to remember. Use the iOS screenshot functionality (press the home button and sleep/wake button at the same time) to capture the note.
Screenshots taken on your iOS device appear in your camera roll in the Photos App. Tap on the screenshot of the note and tap the action button from the pop-up menu bar (located on the far left). Tap "Use as Wallpaper" then adjust the scale so the note is within the center square. Tap "Set" and select to set the image as the lock screen.
Now, when you wake your device, your message will be waiting.
My own spin on this tip is using your iPhone's (or iPod Touch's) camera to take a photo of the thing(s) that you might need to remember--like that nearly empty jug of milk in your refrigerator, the street corner where you parked your car before a long night out, or that piece of jewelry your girlfriend likes so you can get it for her for Valentine's Day this year.
Some users have suggested making the lock screen an "if found" ad. You can use either process described above--create a note with Notes, or take a photo of the info you want to share.
Do you have an interesting use for your lock screen? Let me know in the comments!