I found this article on http://glassyeyes.blogspot.com blog and found it very usefull. So here it goes:

Much has been said about the pupillary distance when ordering glasses online and the difficulty in obtaining it. It’s a very important measurement and contrary to what the opticians will tell you, it’s very easy to get — in the comfort of your own home even!

A few of the sites have methods requiring you to have a friend hold a ruler up at the bridge of your nose or something akin to that. That’s a pain and not very precise.

I’ve got a better way.

Here are the required tools:
1) a pair of your existing eyeglasses (doesn’t matter too much on the prescription — just so you can see through them)
2) a non-permanent marker
3) a ruler (with millimeters preferably*)

How to do it:
1) Stand in front of a mirror (3 to 4 feet back)
2) Close Obscure your left eye with a card or thick paper
3) Mark the center of the right pupil with on the eyeglass lens with the marker.
4) Repeat with other eye
5) Measure the distance between the two dots
6) You’re done

You can have a friend do this for you also, but I had better results doing it myself. It works. I had my “professionally measured” PD from the last time I was mugged at Len Crafters, and it was identical to my latest measurement. Didn’t surprise me at all as this was precisely the method they used to measure it.

Anyway, if the optometrist is giving you hell about getting the PD, tell ’em to stuff it and do it yourself. I think you’ll be happy with the result.

* NOTE: If you don’t have a metric ruler (and if you’re a child of the seventies like me, you’ll never be without one), just pop your standard measurement as such into the google and you’ll get the number you need. Ex. “2 9/16 inches to mm” returns “(2 9/16) inches = 65.0875 millimeters”.