You should always, always always always always always always always always always always always always always always always adjust the idle speed with the idle speed screw, NOT THE TRIM. That way, when you push the brakes you don't kill the engine. I'm sort of peevish about this if you can't tell.

Start by adjusting the throttle trim so when you are at idle and you push the brakes there is no change in RPM. Then adjust the idle screw to set the idle speed just fast enough so it idles reliably. There is no reason you should ever need to use the throttle trim.