1) Easy to say, easy to spell, easy to find.
Who wants to look stupid when ordering or discussing a book? Make your title easy to say. The spelling matters because if someone is searching on Amazon or Google and they get the spelling wrong, then the search engines won't find them. You can help people find your book by using uncommon words. My name, Sarah Duncan, is fairly common. If someone does a Google search for me, my website does come up first, but there are lots of other Sarah Duncans around, as well as things like "...said Sarah. Duncan, on the other hand..." If your title has lots of common words then it's going to be harder to find on search engines.
2) Fits in with the genre
The Guernsey Literary and Potato Peel Pie Society doesn't sound like it's a thriller or teenage vampire book, In Cold Blood doesn't say romance, Twilight couldn't really be anything but a vampire story. Titles need to match the genre. Go into a bookshop and and look at the titles in 'your' section. You're looking for patterns, for example, lots of one word titles or titles which contain place names. Are there puns or plays on words? Slightly risque? Your title needs to fit in here.
3) Has some originality or quirkiness
Would Captain Corelli's Mandolin have done as well as The Italian with the Guitar? I think not. Strong nouns are the answer here. If I say "the book about the tractors", I bet most of you will know the book I mean. Penguin used that line to advertise Marina Lewycka's next book, which shows what a powerful technique this is.
4) Uses 'special' words.
There are some words that have more power than others. Lucky. Secret. Desire. Midnight in the Garden of Good and Evil is one of my favourite titles ever (it's a good book, too), and all those nouns are special words. Numbers and colours work well, although some numbers and colours are better than others - 12 Shades of Beige doesn't have quite the same ring.
Deconstruct some of your favourite and least favourite titles and analyse what makes them work (or not). Then try to apply the same principles to your own. I think the 5th element is time - good titles rarely come easily or quickly in my experience, but when the right one comes along it's easy to spot.