I found that holding the sanding block dead straight in line with the neck was a bit of an issue, so I built this little jig to make life easier.
It is pretty simple, and works well. I can now put most of my effort into simply moving the block back and forth, knowing that the jig will hold it straight.
There are small blocks in the bottom that hold the neck straight and square. There are also blocks on each end to hold it from moving.

Here's a shot of the sanding block in the jig.

And one more.
