You probably shouldn't downshift as a way of engine braking if the tranny will take care of it for you. Plus the brake pads are cheaper to replace...
Downshifting for the sake of accelerating is a different story. I have found that if I keep the throttle relatively static and down shift, the throttle blip combined with my steady foot results in a really smooth shift and sets me up to plant it when the time is right. This is counter intuitive to most drivers since your instincts are to lift off the throttle since it will SRM on it's own. The throttle position varies depending on what RPMs and what gear you are in when you make the downshift. I have more of a feeling/connection with what's right by now and love how it feels to drop a gear and then seamlessly transfer to warp speed.
