Gridskipper has a nice round-up of techniques for sleeping on a plane.
Avoid the last row of the plane, any seats in front of the exit row, bulkhead and aisle seats in favor of window seats. Take one carry-on. Skip the coffee. Board early so you can nab a blanket and pillow. Bring a neck pillow from home. Wear shoes you can slip off easily. Wear ear plugs. Take sleeping pills if you please. And feel free to recline your chair back, but don't be a wanker about it.