Well it depends how you like to drive... Just like in real life, down-shifting while braking will make braking easier (since you also use engine to brake), but make sure not to hit limiter when doing it (well hitting a limiter when down-shifting in gt5 engages clutch so you no longer do engine braking until revs drop to its working range). I like to go down to about 4,5k rpm before going one gear down, but I bet you that after 20-30 minutes of practice you will do it instinctively (prop. spelled?) just by hearing how high engine goes.
