1) Try to take them during the day, if at night, get as much light in the picture as possible.
2) Take them down the center of the cabin, now down an aisle or lined up with a row of seats. If the seats are off-balance, 2-3 or 3-4-2, they will look much better actually off-balance than with the entire fuselage shifted just so that you end up still in the aisle.
2b) On widebodies, it looks better to take the pic. across the seats, where I've got Xs: 22-55X55-22, or 333-44x44-333.
3) For upper-deck 747 shots, get as far back as possible if you're photographing parallel, and be sure you're centered on the deck. Try to kneel just a little to give the deck more height, unless you are very short.
4) If you're taking shots against the flow of the aircraft, get back from your target, if it's a row of seats (a bench of 2, perhaps, on a 2-5-2 jet), back into the seats beyond, don't stand in the aisle. Use the aisle space in the picture to open it up.
5) For in-flight pictures, follow these same guidelines and take pictures from aft just to be on the safe legal side.
6) Aircraft ceilins are very artistic and flow smoothly usualy, so include the cabin's top in your picture.
7) Airliner brochure's sometimes show pictures of a bench of seats, but nothing in front of them! This doesn't happen very often in real life, there's always another row or a bulkhead, so don't try for this. If you want to show the arrangement of seats, climb up on an armrest when no one is looking, get as high as you possibly can, and shoot down and back, so that the rows are clear and parallel (definitely from the center) and the seats can be easily counted.
7b) Shots taken at odd angles make it difficult to count the seats because they don't appear to line up, so viewers can't tell how many seats there really are, and this is not aestheticaly pleasing.
8) Close overhead bins and open windows before photographing. If most bins are open, open the few that aren't, just in your shot.
9) For cockpit photographs, don't turn, but make sure the 'dashboard' is level with your viewfinder's bottom and top, and get as centered as possible even if this means moving into the cockpit a ways. Include as much of the instrument panel as possible, but maximize window space at the same time.