To maximize the braking power of the car, you want to keep the revs as high as possible as that way the engine is helping to slow you down. You'll learn how low to let it drop in each gear so that when you shift it will go up to nearly max RPM. In most cases the gears tend to be about equal so it's about the same RPM each time you shift, but it may vary depending on how customized your gears are.
You also will want to drop a little lower when shifting into second and ESPECIALLY if dropping all the way to first as the lower gears will tend to make the drive wheels want to lock up if you shift into too high an rpm. Lifting off the brake a tiny bit just as you shift helps too. Blipping the throttle yourself, even with the DS3, seems to help as well, the auto-blip isn't quite right. If you use the stick for gas/brake I guess you can't, but it works with the triggers. You'd like to have the it at the RPM that it will go to in that gear before the shift finishes. If the drive wheels lock, give it a bit of gas.