八 is two strokes curving away from each other — the picture of something being DIVIDED or split apart. That was its original meaning; it was then borrowed for the sound 'eight'. You can still feel the two halves separating.
八 is the luckiest number in China — it sounds like 发 (fā), as in 发财 ('get rich'). People pay fortunes for phone numbers, license plates, and addresses full of 8s. The Beijing Olympics opened at 8:08:08 pm on 8/8/2008. No number is more sought-after.
Two strokes splitting apart — 'divide', borrowed to mean EIGHT.
八 sounds like 发 (fā, 'to prosper'), so it's wildly lucky. The Beijing Olympics deliberately opened at 8:08:08 pm on 08/08/2008 — eights all the way down.