The absolute best way, is for you to do it. When you are at home, start a hot shower in your bathroom and wait for it to get nice and steamy. This will prevent any dust from floating around. Then just take your time and do some practice runs to see how the protector aligns up, then put that sucker on. If you do get dust under the protector, get 2 pieces of masking tape and use 1 piece to lift the protector and the other piece to slide between the screen and the protector in order to fish the dust out. I've always had good luck using this method.
I've always had horrible experience at any retailer letting them install the protector.