many people downsift to take advantage of engine braking. Enging braking helps decelerate the car while saving some brake wear. The downside is that if you slip the clutch on the downshift you are trading brake wear for clutch wear. I think we all know that clutches cost more than brake pads.
The best (and most fun) way to downshift is to rev-match on the downshift. Heel-toe downshifts match the engine speed to the transmission speed and will almost eliminate the wear on your clutch. There will still be some slipping as the clutch speed will have slowed while the clutch pedal is depressed, but it will be minimal. See detailed instructions at this website:
http://www.edmunds.com/ownership/how...2/article.html