Slytherin makeup and jewelry ideas?

Jewelry : everything based on silver and green (emeralds, though these are expensive !), but you may try semi-precious stones like malachites or turquoises.

I recently bought earrings made up of silver with green enamel - the Slytherin set of colors.

As for make-up, I'm not into this. Just keep it simple, in general. Too much would make you look like a prostitute - well, that's my thinking...

Jewelry - Trendy Snake-themed rings, necklaces, bangles, whatever you can find (just make sure it's all close to the same - try not to mix silver and gold color. Also, only choose a couple accessories so your costume doesn't seem forced.)
- OR -
Any subtle green and black jewelry you can find - go for classic looks (wizard fashion, as in the books, develops slower than Muggle fashion does). Nothing too flashy, think of words that define Slytherins: Powerful, High-class, Ambitious, Confident.
Makeup -
Darker, but not extreme. Exaggerate your eyes with black or different shades of brown. Choose a solid red or darker shade of lipstick that won't make you look older than you are, but adds a sleekness/sharpness to your look.

Hope this helps!
(Oh and I just added an example of the kind of snake-themed jewelry you might want to wear)