Hi there Mike,
Basically all I can say is use a high-ish ISO, try not to use the flash, and also remember to stop down to as low as you can, as BA747-436 says f5.6 for cabin should be just about right.
Since you have the 300D, I would suggest shooting in "Manual" mode to get the correct exposure level. Shoot at f4.0-f5.6 then adjust ISO to 200-800 should be sufficient, then adjust shutter speed as required. (For Cabin shots)
Window shots are really quite easy, I usually stop down to about f7.1-f11 and use exposure compensation if necessary. I will tend to shoot at ISO100-200 because the light you get up there is quite good. Also I would recommend that you try and plan in advance where to sit, and where the light will be best for you!
As for the cockpit. Always always ask if it is okay to take some pictures, I wouldn't want to upset my pilot! Maybe ask one of the cabin crew at the start of the flight if it is okay to come up and have a look once on the ground at your destination. If not as you are leaving the aircraft ask on the way up to the front. As far as settings go, usually I would shoot in "Manual" mode to get the correct exposure. Typically I would aim for f4.0-f8.0. It really does depend on the conditions up there! I would use a bit of flash, but under-expose the flash quite a bit, -1 stop maybe?
Don't be afraid to ask any more questions, there are many people here that want to help out, including myself. One final note, don't be afraid to take masses of pictures on board the plane! I always find I don't take enough, so everytime I fly I will literally fill up a card if I want.
Good luck mate and happy contrails.