If your bike computer is like most (and you do have a bike computer by now, don't you?), you had to set all sorts of mysterious numbers before you used it for the first time. (You did set all those settings, didn't you?)
One of those settings was probably the circumference (distance around the edge) of your tire; the computer calculates your speed by counting the number of rotations and using the size of your tire to figure out the actual distance covered. That number varies by the size of tire you currently have installed -- a 700x25c tire has a slightly different circumference than a 700x28c tire. The manual for your computer probably has a table of tire sizes and corresponding circumference numbers. It's usually a four-digit integer, so it's factored strange in order to be a whole number that you can easily enter into your computer with the buttons.
Here's the rub. If you're like me, you tend to forget your circumference setting. And when your computer's battery dies or you just accidentally reset your computer in the middle of a ride (it happened to me earlier this year!), your computer no longer can accurately measure your speed or your distance. And if you're all nerdy about numbers like I am, you suddenly feel naked during your ride.
So ... with all that backstory, here's the tip. Write your magic circumference number, and tape it inside your computer, on the inside of the removable cover for your battery. (Don't put it on the battery, because you'll eventually replace the battery.) That way, if you ever need that magic number in the middle of a ride or when you don't have your computer's manual handy, it's right there for you.
As you can see, my magic circumference number for 700x25c tires is 2146. So if I forget it, now I can just ask one of you instead.