Over time I've changed my opinion on this. Right now I think I prefer:
1. Some character description, not too detailed (well, that depends on genre)
2. Description given early (so the reader doesn't form a certain picture, then get disoriented when you provide a different one late in the game)
3. Describe the character once. You want to point out the character has pale blue eyes? Great. Do so. But don't remind me of his pale blue eyes, his ice blue eyes, his sky blue eyes and his light blue eyes ever other page, or I will hunt you down and kill you.
4. The above goes for hair as well. Describe it once, not four times in every freaking scene. Unless your character is Rapunzel and her hair is crucial to the plot, drop it. Hair is boring.
Oh, and these days I'd rather you just said "She had curly brown hair" rather than "she reached up and swiftly tucked one of her thick brown curls behind her ear, thereby giving me, the author, the perfect opportunity to mention what her hair looks like in show, because everyone keeps telling me 'show don't tell.'"
This show-don't-tell character description stuff is turning everything into a tedious game of charades.
Man, I'm cranky today.