For how prestigious you look- it it based off how much you are wearing, the type of clothing you are wearing, or something else?
For instance, does a "Lovely silky shirt" which takes 10 silk, and 3 silver to make give more to prestige then "Tattered Leather shirt?" I've been told it's based off how much clothing has been mended, but if I go out in full hobo gear I should hope I look poor :S Anyone know what determines it?

It's actually simple.
Normal clothing, jewellery and armour give prestidge - it's dependant on the item type, but not a particular design. Necklaces up your prestidge more than rings, but a roughly-hewn wooden ring (with splinters!) gives you the same prestidge as a gold and diamond one with angel hair, demon skulls or whatnot.
Clothing gives the same prestidge no matter how many times mended. HOWEVER, when it is mended one time too many, it changes into rags. Rags (tawdry clothes, etc) reduce prestidge instead of increasing it.
Prestidge affects influencing in a small, but noticable way. For example, I cannot influence some peeps in Estelbar with the max prestidge (royalty), but I can when I remove frogges. For some reason, they give a lot of looks.

Ah, and removing all clothes doesn't work. It's better to remove almost eveything but keep an undershirt or so.

So, basically. If you want to beg, wear tawdry things (expensive, because mending costs, so may not be worth it) or almost nothing. If you want to influence in another way, pile up on clothes and bling.

I have also found that you also need a combination of jewelry and high fashion to get up to looking like royalty (Unless you are a trans tailor, then you only need your robes to do it). Having just one or the other will only take you so far.

Anyway, I've heard many times that if not wearing anything but boxers () (you look nondescript) and looking like royalty doesn't change anything for charity attacks. Obviously, for the rest of the attacks looking like royalty is great.

But I can never find rags.

I'm not sure there's no difference between individual designs; I'm pretty sure that mending always reduces prestige, and more expensive designs usually take more mendings to get down to rags.

Nope.

I once asked the Charites if it was possible to design clothes that were purposefully tattered, so that they'd start out with a negative prestige, and they told me that it wasn't, because all individual types of clothing have set prestige and it doesn't vary based on individual design.

Shhhh, are you informing customers? Shame on you. SHAME!

I'm not a tailor, and my jewelry and clothing alone gets me to royalty. A cloak, a coat, a greatrobe, a crown, and countless rings.

You are a bit under-dressed, and utterly satiated.
You are fully rested.
You are sober and in control.
You are completely sane.
From the clothing that can be seen, you look like royalty.

Yep, I get the same thing. I'm a bit undressed and yet I'm wearing:
iridescent butterfly panties, a mist-emblazoned ring of smoky quartz, an onyx and moonstone stud, a circlet of golden chains, a necklace with a dangling golden seahorse, a silver bracelet set with moonstones, a sheer black tinted scarf, a pair of black webbed stockings, a set of flowing smoke grey robes, a butterfly bra, a black velvet backpack, a pair of black velvet shoes, a pair of fingerless black gloves.

Underwear, robes, stockings, shoes, gloves, scarf...the only reason I'm not wearing a hat is because of the circlet and no cloaks over robes; I'm sorry. But, geeze..that's a lot of clothes.

No shirt, no trousers. I dunno, while you're right and it does seem a bit off at times, shirt and trousers tend to be important. Either that, or also wear a cloak and a coat.