Hmm.... the vision you get of a "head on a spike" is similar to what it would look like with Bruce's idea, but once I took a second to think about it, it seemed unlikely that the spear would penetrate that far.
So, I crossed my fingers, held my breath, and braced myself for a Google image search....
From what I found, it seems like it can go both ways. So if you want the easy way, just perch the head on top of a spear. The angle resulting from it not "connecting" properly will probably be more realistic as well, since I'm sure that the spear wasn't often inserted perfectly straight.
For a more theatrical look, Bruce's suggestion is probably pretty good. You could also use one of these inserted in the top of the head, maybe with the hilt cut down and shaved off so it just looks like the point coming through:
EDIT: I didn't know this, but Bruce's harpoon idea became better as soon as I found out: harpoons come in flat silver, which will match the "shaft" bar a lot better than the chrome I was envisioning.