Replacement headlight bulbs. The following are known to work:
Eiko 6260HD H4 Heavy-Duty 60/55W
Honda 34901-MS2-671 45/45W. This is the OEM part for US and Canadian 2003s.
Honda 34901-MS2-672 45/45W. This is the OEM part for US and Canadian 2004s.
Honda 34901-MC7-601AH 60/55W. This is Honda's "super secret" replacement bulb.
Osram Silver Star H4 60/55W
Sylvania Vision Plus H4 and H4ST 60/55W. The ST version is ruggedized and better suited for use on motorcycles.
Adapter shims. The ST has a proprietary layout for the three tabs around the outside edge of the mounting flange. These correct that problem. They also give you more options if you have to replace a bulb on the road.
The OEM bulbs ('671 and '672) do not require shims.
Cotton gloves for handling the bulbs. Skin oils on the envelope of the bulb will create hot spots that can cause it to break.
If you do touch the bulb, clean it thoroughly with denatured alcohol and a lint-free cloth before installing it.
Put the bike on the centerstand.
Turn the handlebars to full lock on the side of the bulb being replaced.
Press the side tabs on the 3-pin connector at the back of the bulb and remove it.
Remove the dust cover from the bulb.
Disengage the retainer by pushing in on it and squeezing the end out from under the brass plate.
Remove the old bulb.
If using shims:
Bend back or cut off the two lower tabs on the bulb.
Bend the upper tab back a few degrees.
Slide a shim onto the bulb, making sure it seats fully against the base.
Insert the bulb into the lamp housing, making sure the tabs are lined up with the slots.
Reattach the retainer.
Install the dust cover, making sure it seals all the way around the lamp housing.
Install the 3-pin connector.
The US DOT wattage limit for these bulbs is 60/55W.
Higher wattages will work but may require rewiring to accommodate the extra current. There have been no observations of whether or not the additional heat has any ill effects on the lamp housing over the long term.
A more drastic replacement option is to replace the entire headlamp unit with the one specified for the non-UK European models: 33100-MCS-G01 (includes adjuster and bulbs) UKP197.39 from David Silver Spares in the UK (this price is with the 2-3wk delay discount, which may not be offered unless you ask). The Euro-spec headlamp unit is probably not 100% legal in the USA, but regular H4s do fit perfectly...