One of the features of the Canon 40D often overlooked by photographers is the ability to register (or preset) camera user settings. This is an incredibly useful feature for nature photographers as it allows you to recall several  camera functions at once from the exposure command dial.

We have all been caught taking a photo of a static object such as a plant or sleeping duck  where we use low ISOs for quality and small apertures for large DOF often with mirror lockup if the light conditions require slow shutter speeds,  with camera on tripod when a bird flies over. Though it is often possible to whip the camera off of the tripod quickly, the camera which was set to static mode shooting requires several changes to get it into "action\flight mode" and this would take tens of seconds normally.  With the camera registered user settings wholesale setup changes are much faster!
 

 
I have  "registered" the C1 setting to be my flight or action setting as it is easy to remember that  "one" is for action, while C2 is for static and C3 are for Warbler (small birds perched) shooting. The nice thing about the presets is that any and all functions can be overridden. For example if there is heavy overcast and I use  C1 which I have preset to ISO 400 which is a sunny day setting and may not give a correct exposure with the f-stop and shutter speed I choose to use, I simply set the ISO to 800 or higher.  ISO 800 will be retained until  C1 is deselected at which time it defaults to the 400 ISO setting. The same goes for shutter speed and aperture which I often change to compensate the exposure.

The only other important parameter that cannot be set for flight shooting is the lens focus limiter, I always use the far limit for flight. It would be very useful if Canon can make this possible from the presets in the future.

Your own settings may be different than mine depending on the type of photography you do.

Refer to page 165 in the EOS 40D manual for instructions on registering the functions.