I found these fantastic mirror-nails from last autumn! They made the girls at the MAC counter gasping with enthusiasms and I think they are still cool.
Yes, no polish can do this, it's a sticker from Essence and it can be done in 5 minutes.
Just clean your nails with nail polish remover, so it's fat-free and put the sticker on you nails. Cut the right length with a small scissor and file the rest few millimeters down. For extra shine and protection I used a top coat.
If you're careful with your nails (no house-cleaning, cooking or gardening) it will last up to three days.
While googeling for the stickers, I saw that Ivania from Love Aesthetics got mirror nails at a salon (also last autumn). They probably are better and last longer, but they might be more expensive.
pic on the left via www.essence.eu