I have to disagree with a few things Naseer said. The camera body can make a difference. For example, the added inertia of a heavy pro body can keep it steadier and the vertical grip and larger body can make it easier to maintain a solid grip on the camera. Also, Nikon makes an equivalent to their pro full frame lens (24-70 f/2.8) for their APS-C cameras (the 17-55 f/2.8), so in some cases at least, you can get pro glass for the non pro bodies. Also, the 70-200 f/2.8 works fine on their APS-C cameras. Either way, we don't know what gear you have, so I won't continue on here about that.

Next, better than shooting through a clear window, find a plane where you can open the window or even remove the door. This will be the case in some small planes and you won't be at an altitude where the plane will be pressurized anyway. It's an option you should keep in mind. Maybe better, shoot from a helicopter. Many chopper have windwings (like some planes) you can open for ventilation and you can shoot through those, or remove the door altogether.

For the absolute best picture quality, you may want to do things such as shooting at your camera's base ISO (e.g., ISO 80, or 100, or 200), and the optimal aperture for the lens you're using (e.g., f/5.6 or f/8). In these cases, you might end up with a lower shutter speed than you'd like, especially when you consider vibrations from the engine, or shaking from turbulence. In this case, you should consider using a gyroscope.

http://www.ken-lab.com

If the photography is for documentary purposes, then all of this is probably overkill. If it's for artistic purposes, you'll want to seriously consider all of the above (and Naseer's suggestions).

Oh, one last thing. If you're shooting through glass, use the polarizer to help reduce reflections. I'm not sure how this will work with the plexiglass windows of aircraft, but keep it in mind.

I have to disagree with a few things Naseer said. The camera body can make a difference. For example, the added inertia of a heavy pro body can keep it steadier and the vertical grip and larger body can make it easier to maintain a solid grip on the camera. Also, Nikon makes an equivalent to their pro full frame lens (24-70 f/2.8) for their APS-C cameras (the 17-55 f/2.8), so in some cases at least, you can get pro glass for the non pro bodies. Also, the 70-200 f/2.8 works fine on their APS-C cameras. Either way, we don't know what gear you have, so I won't continue on here about that.

Next, better than shooting through a clear window, find a plane where you can open the window or even remove the door. This will be the case in some small planes and you won't be at an altitude where the plane will be pressurized anyway. It's an option you should keep in mind. Maybe better, shoot from a helicopter. Many choppers have windwings (like some planes) you can open for ventilation and you can shoot through those, or remove the door altogether.

For the absolute best picture quality, you may want to do things such as shooting at your camera's base ISO (e.g., ISO 80, or 100, or 200), and the optimal aperture for the lens you're using (e.g., f/5.6 or f/8). In these cases, you might end up with a lower shutter speed than you'd like, especially when you consider vibrations from the engine, or shaking from turbulence. In this case, you should consider using a gyroscope.

http://www.ken-lab.com

If the photography is for documentary purposes, then all of this is probably overkill. If it's for artistic purposes, you'll want to seriously consider all of the above (and Naseer's suggestions).

Oh, one last thing. If you're shooting through glass, use the polarizer to help reduce reflections. I'm not sure how this will work with the plexiglass windows of aircraft, but keep it in mind.