Since these little things annoy me so much, I decided to write a small little guide on ebay to save some other people a little hassle.  Click here:
http://reviews.ebay.com/Removing-2-Layer-VOID-Rental-Stickers-on-Vintage-Games_W0QQugidZ10000000002348616

Hopefully this will help some of you out, these things are sooo hard to remove sometimes.  Let me know if you all think this is helpful or not.  
~~NGD

I heard you describe this method before somewhere on the forums but it's cool to see it written out in detail.  I voted the guide "Yes" for helpful.

Nice man, i have such a hard time removing these. They are one of the hardest to get of.

I heard Goo Gone works wonders - they sell them at the Dollar Tree not so far from my house. :o

I have used goo gone before, and it doesn't work nearly as well as this method.  Goo Gone is good for other kinds of stickers though, just not these.
~~NGD

i used goo gone once and it left a weird stain on the game, id say dont use goo gone or goof off or anything like that it may be too strong, they way he describes is how ive ben doing it and it really works good

thanks for writing that NGD, I hate those stickers. Now I don't have to replace some carts now.

I've had luck with getting these rental stickers off of carts with goo gone, but this method seems just as good.  What I really want to know is how you can safely remove rental stickers from disk games.  I just picked up 3 GC games that have rental stickers almost the same size as the disk, they look terrible.  Has anyone tried this method on CDs?  If it works I'll head out and try my hand at it today.

YES! Thank you so much! I was looking for the best way to get rid of a peeled "VOID" label on my Mario's Time Machine game. Thanks so much man!

i use goo gone to take rental stickers off and they come off super fast. ive also never had them leave a stain... a bit greasy but i just wipe it down with a dry paper towel afterwards and its all good.