As I use a 300D myself, I'm familiar with your problem!
At the beginning of a spotting day, take a few images of "usual" aircraft ( Lufthansa 737s? ) and check your settings!
Zoom into the images on the LCD
screen and then adjust the exposure. Do this, whenever light changes... like clouds in front of the sun, etc...
Regarding the editing... Adobe Photoshop CS
has a nice feature, only know the German name, but shouldn't be a problem for you.
Go to Bild - Anpassen - Tiefen / Lichter...
With the Lights feature, you can turn an overexposed image into a normal exposed one, but the quality will drop the more you use it!