The VAR 13 ([pictured below on the left) is a great tool for the job (I have used it successfully with old Bianchi built-in headsets), but I don't think it is made anymore. They can be had on eBay, but are often spendy. [utag=458869]WilliamT, if you buy one, make 100% sure it has the two little screw-in pins. They can moved from being parallel (for adjustable BB cups - you can see the holes for this position in the photo) to pointing towards each other (perfect for the little holes in the headset cup/race and knurled top nut.) The tool is worthless without them. The wing nut allows for a wide range of adjustability.

VAR also made a purpose-designed set of pliers to use on the knurled nut while the VAR 13 was used on the cup/race. The pliers are pictured below on the right. I've used those, too, but they really aren't any better than using a pair of channel-lock pliers (with a rag to avoid leaving "scars"). The strap wrench shown by iab would probably work at least as well with less risk of surface damage to the knurled nut.

I think the best thing about the VAR tools actually is their appearance. Their is a certain Gallic, funky, almost rustic charm to them, at least to my eye