Always ask if they can give you a free upgrade. I've been given one a couple of times because I've always asked politely, and explained that I cramp easily in narrow seats. Sometimes I'll get a "I'll see what I can do" and if the plane is not completely booked, I might be given a seat with extra leg space, economy plus, or even business class.
I used to fly between Japan and America constantly, and I've been given a business upgrade twice simply by asking. I seem to have the most success with Delta and ANA.
Always ask for an upgrade, especially if you're above base level on their rewards scheme. I got heaps of Emirates upgrades to BC doing this. Also, I once had my seat double booked on an a380, was a great economy with good leg room. If this happens, and the plane is at full capacity, don't fight for your seat. Arrived at seat, taken, check boarding passes, double booked. I told the lady to keep the seat, called a flight attendant over to sort it out, BC upgrade right in front of her!