Because all the seeds
of a butternut squash
are in the bulbous
end, you can cut it in half right where it starts to bulge out. You will then have one completely solid, cylindrical hunk of squash, which is pretty easy to peel and cube, and a hollow hunk that you should split open to remove the seeds and then slice up.
I mention this because if you are in a hurry and don't need to use up the whole thing, you can just use the top piece, which is quick work to prepare (peel and chop), and put aside the hollow piece, which is slower and gloppier to prepare (split, seed, slice, peel, chop), for later. If you are up to it, you can even peel the seedless piece with your chef's knife by standing the squash section on its cut end and taking the peel off in thin vertical slices.
Because it's less complicated to prepare the seedless "neck" of the squash, you can sometimes find a variation on butternut called "neck pumpkin." This resembles a butternut squash crossed with a giraffe; its bulbous, seed-bearing section is about the same size as a butternut's, but the seedless neck is greatly elongated: about three or four times the length of a butternut's neck. Usually the neck part is curved, so you can't do the vertical peel w/chef's knife trick on it, but its high ratio of squash-to-seed makes it pretty popular among people needing large quantities of prepared squash for making pies and such. I've never seen this in the grocery store, though. Look for it in farmer's markets, mainly in the autumn.