I usually start with a polite hello. You can usually tell very quickly if someone wants to chat or not. Over the course of 20 years, I've had some great conversations on planes.

Somewhere long ago I read an etiquette piece that suggested if your seatmate had a magazine or newspaper, he *may* be open to conversation, whereas a book reader typically wants to be left alone. Probably a moot point in 2014...these days it's more like earbuds vs. Beats by Dre...