Downshift if you need to accelerate fast and ur rpm's have dropped. Also, don't downshift with the intent for it to slow you down.

Don't downshift to slow down, but downshift AS A RESULT of slowing down.

what i mean by this is dont 'downshift to let the engine do the braking for you. Use your brakes to slow down, but downshift as you use your brakes so that you are in the proper gear in case you need to accelerate once again.