The first time we were upgraded was on a visit to the US and they asked for volunteers to fly the next day. They gave you cash and an upgrade to business. It was US Airways so business class was nice but not luxurious.
We have flown Emirates (my very favourite airline) to Australia, Uganda, South Africa, and Thailand. So we earn airmiles and now we've finally got an Emirates credit card that gives us airmiles. These are all long haul flights on Emirates. Never experienced first class but business class is awesome. Lie flat beds, an option of a mattress added to it. Tons of room, a bigger tv screen, really good food and wines and champagne. Real dishes, a tablecloth on your tray. When you order a drink they also give you a dish of warm cashews and assorted nuts. A dessert cart comes around your meal with cheeses, desserts, chocolates. I love it so much I don't want to get off the plane! We don't have enough airmiles to upgrade on our next trip but we should for the trip after. Sigh.