I use the number system to record blocking. Like smejs, I also have my script page on the left hand side so I can have a clean page on the right (I'm right handed).
It's also nice to make a mini copy of your groundplan and to photocopy it on the top of your blocking notation pages. You can mark starting/ending points of your characters for each page of dialogue. That way, when the director says he wants to pick up on page 68, you'll know exactly where everyone should be onstage with just a glance at your book.
I usually skip some numbers when taking blocking to allow for inserts of new blocking. For example, if I take a note of blocking that is near the top of the page, I might label it number 2. If the next piece of blocking is near the bottom of the page, I might label it number 7. That way, if things get added before, in between, or in the middle of those numbers, I've got some leeway. I think of my pages in counts of 10 for blocking, if a note is taken near the middle of the page, that's in the 5 range, at the top is 1 and at the bottom 10. I hope that makes sense.
There are a million ways to do the number thing though. I know people who do things in multiples of ten, so they might have a number 20, then the next note is number 30... if something gets added in the middle, it's 25, etc. I've also seen people who just number in order when they take the notes and if something needs to be added, they mark them 1.1, 1.2, 1.4, etc.
I agree with ChaCha though, I really don't like taking blocking.