I'll be going on tour soon and I'd like to be dressed to the nines - but... what the hell does that mean? Well, great question. I can't find a really satisfactory answer. What being dressed to the nines means is that you are dressed very nicely, decked out for a certain occasion. But that's pretty much where everyone stops agreeing about this particular phrase.
One theory is that it's a bastardization of the pronunciation of being dressed to thine eyes, but I personally can't really see how being dressed right up to your eyeballs looks good on anyone. Certainly not enough for it to have slipped into a misheard common usage.
Another origin tossed about frequently for this phrase claims roots in the fantastically sharp uniforms worn by the 99th Lanarkshire Regiment of Foot, which was raised in 1824. They were all brass buttons and fire-engine-red wool. And while I'm sure they were damn sexy, most people can't help but point out that there's a pretty big difference between 9 and 99.
The best answer I could come up with is that the number 9 has always been viewed as a mystical number. It is viewed as the number of perfection, with many religious connotations and literary references sprinkled all over the place. To me, this answer makes the most sense. Because to say, "Wow, she's dressed to perfection," makes more sense than, "Wow! She's dressed up to her eyes. Well, what I mean to say is that she looks as good as an nineteenth century British foot soldier."