It depends on your skin tone and what you're wearing. I split my outfits into "earth" and "metal" tones.
For earth tones, I use a really thin line of a dark brown shimmery shadow right on my upper lashes, and just over my lower lashes, spreading right out to just past the lashes to make them look longer. For the rest of the eye, I use a lighter coloured shadow, like a darker beige or an orange. The Body Shop does an awesome shimmer cube that has four great earth tones in it, I could live pretty much just off that.

For metals, I love a black liner and a mauvey-grey shadow. I have blue eyes, but because of my skin, which is a bit orangey, anything too blue makes me look like I've been punched. But that tends to be a night time look for me.