if your using .PNG (i recommend you do) make sure you have a 350x280 shape for the background and make sure it is perfectly aligned to the canvas (you can do this automatically in illustrator, not sure about corel) and yea as asti said, make sure the background shape doesn't have a stroke on it
i find it easiest to just keep a blank file called "LT template" with the background shape locked in place
hope this helps!