You will need to look at your photos using software like photoshop. There are a number of very basic things you can do:
(1) Levels - it is the way the bright and dark pixels are distributed. You can adjust the level to get a better level.
Some references:
http://www.twistedtreephoto.com/ ... op%20tutorial1.html (a good one)
http://www.digitaldiver.net/lib_docs/Levels_Basics.pdf
(2) Shadow and higlight - you can brigtehn up shadow area.
Ref:
http://www.digitalproducer.com/2 ... scsshadow031028.htm
(3) Saturation - increase the colour saturation of some colours.
Hope this helps
It is of course important to get the shot right with the camera - 400D is a god camera capable of getting brilliant images. Post processing helps to bring out the best of the images.
Happy snapping!