Ikea's large Expedit bookshelf can be very hard to assemble. Here's a trick to it.

[edit] Steps

Before you start, make sure you have a large, strong cloth on the floor to build the shelf on. I like this because it stops scratching of my hardwood floor and you can use it to move the shelf into place once it's built. And it's NECESSARY for the crucial step below to prevent chipping of the shelf.
Follow the instructions until you get all of the shelves and internal supports doweled together.
Once you get all of the shelves in place, DON'T screw any of the remaining corners together until you get all of the dowels roughly lined up with their holes.
Line up and gradually tighten two more corners (so you have a total of three corners screwed together).
- If you can't get the last corner to join, do this: With two people (twisting the shelf at this point could be fatal) lift the entire bookshelf up onto its side so that the loose corner is on the floor. This provides enough weight so that the screw holes should line up.
Put in the last two screws (There's not enough room to turn the little Allen wrench in a full circle. You have to keep taking it out and re-seating it. By the time I got to this point I was giddy that the screws were going in at all...).
Tighten all of the screws down.
Carefully lower the Expedit onto its face again.
Raise it upright again, this time onto its bottom.
Enjoy your beautiful new piece of Swedish Modern furniture.