Text messages are virtually impossible to ignore. They pop up on your phone’s screen, making a sound, vibrating, or both. However, text messages are great for reminding you to do stuff.
Oh, Don’t Forget is a web service that allows you set up a reminder to text your phone at a designated time. (You can also set one up that goes to someone else.
All you do is enter your cell number, the date and time you want the reminder, and your message to your self. No needs to sign up for anything, register for anything, confirm anything, or pay anything. It’s free (though standard messaging rates apply).