Frost is caused by only one thing ... humidity entering the refrigerator and then icing up on any surface it touches. The question is, where is the humidity entering? As stated above, every time the door opens it's letting in humidity, and if the seals around the door are not sealing tight, humidity can enter.
You can't necessarily control the first culprit, but the second may be fixable by simply washing down (very thorough and in great detail), the seals, getting every speck of gunk off! And then cleaning the spot where the seals adhere to the door frame area. Clean is your best friend!