Well this Youtube tutorial shows you how to 'correctly' open a deck of cards. However stupid this sounds, this gives the back of the deck box an unopened look.
Hope this helps.
http://www.youtube.com/watch?v=CDqBS7GQ-jA
Nah, Just kidding. Here is the real one.
http://www.youtube.com/watch?v=lk-Xalr8IlA
I'm reluctant to click on the first link, honestly I'm expecting another Rick Roll.
The way that this person did it in the video is great I'm guessing, (I havn't tried) what I would recomend is to remove the seal compleatly and as someone said deck seals can be purchased at Ellusionist. If you don't want to go out and buy the seals than only one short side of the seal can be carefully peeled up, the tab can then be opened and then resealed.
With the cellophane on a deck the top is almost folded over like wrapping paper on a present. If you unfold the top the deck can then be slid out maybe and inch, from there the tab can be opened. When the deck is slid back in the top of the cellophane can be resealed with a thin strip of double sided tape.
I didn't have to much time to type this out so I'll admit it was a shoddy description, if need be feel free to PM me where I can go into more detail.