Keep the rep timing slow and control the weight throughout the set.
Do not "bounce" the bar off your chest.
Dont lock your elbows out at the top of the movement.
And finally, use a wider than shoulder width grip so that the emphasis is placed on your chest and not your front shoulder muscles.
Instructions
Set up for the decline smith machine bench press by placing a decline bench in the smith machine, lowering the bar and adding the weight you want to use.
Lay back on the bench so that your lower chest is level with the bar. You feet should be firmly secured on the bench padding for support.
Grasp the bar with hands at wider than shoulder width. Unload the weight.
Slowly lower the weight down until the bar is about one inch off your chest.
Pause, and then slowly raise the bar up as far as possible without locking your elbows.