Second of all, Give her some good armor so she won't be using the original stuff when you steal it off of her.
You need 100 level pickpocket with perfect touch and at least level 2 of 5 to get her heavy armor off. 4% to steal took about 15 tries.
It's all about the shield. Let her keep the original shield, but pickpocket everything else. I gave her ysolda's outfit to wear and she does.
Only give her 2 handed weapons(Great Swords, Bow). Take away anything that would allow her to pull out the shield (daggers, maces, regular swords).
Put a nice necklace and ring on her.
Now when you send Lydia home, she will be waiting for you there in her underwear.
When you go home and tell her to follow you, she will be dressed when you exit your home.
It's a little glitchy to get it right, but once you get it, it's done.
Don't know if this works on the patched versions or other characters and all that but I killed about 3 hours fingerin this out.
This is for Xbox360 btw. Didn't mean to post it here but oh well.