First, shake all of the crumbs down into one corner of the bag.
Second, use a pair of scissors to cut that corner off the bag.
Now you can easily dump all of the crumbs into your eager and waiting mouth!
 I'm reminded of this Pepsi commercial.
 This technique should work for other hard-to-finish-the-bottom-of-the-bag items, besides chips, such as Cracker Jacks, etc.