I've posted dozens of answers in this site saying that welding galvanized steel is not a "good engineering practice". However, if you definetely need to do it, here are some recommendations:
a) first of all, burn carefully with a torch the zinc coat located within 3 inches at either side of the weld bevel. In doing this, utmost care must be taken in order not to breath the poisonous yellow fumes (zinc oxyde) emitted. Use a mask and vent the fumes away.
b) Clean carefully the weld bevel with wire brush, either manually or using a motor driven one.
c) Apply the weld.
d) Once it's cool, apply a product called "cold galvanizing", following the manufacturer's instructions as to quantity and thickness of each hand. And here comes the "bad engineering practice". In fact, the so named "cold galvanizing product" doesn't galvanize anything at all. It's nothing else than a common paint rich in zinc powder, whose results can not be considered as a true galvanizing.
Anyway, if it suits you, go ahead and good luck.
Giovanni S. Crisi
Sao Paulo - Brazil
P.S. The good engineering practice says that galvanized structural steel should be joined by bolting, not welding.

BankerQC,
Please heed Professor Crisi's advise, especially paragraph (a). It is very important that the galvanizing is burnt prior to brushing. It's been my experience that many times, welders will try and remove the galvanizing by grinding alone. This will not accomplish the necessary results.

Some years ago I had the opportunity to be involved with the first large galvanized structured chemical plant in mid-Michigan. During the project, pipefitters tried forming 3/4" U-bolts with galvanized rod (non-threaded). They would grind the area (complete circumference) of the rod to be heated then heat to about 1200 degrees or so and bend over a 6" NPS mandrel. The rods ALL would break with less then a 60 degree bend! They had not read my procedure for miscellaneous welding of the galvanized steel components which required not only a case by case authorization, but burning was the first step when allowed. I had the fitters follow the "burn first" paragraph of the procedure and required them to wire brush afterward. Literally hundreds of U-bolts were made after that simple change with NO failures. I had a fitter literally bend a scrap rod 360 degrees after the galvanizing burnt off and brushed prior to heating.

A secondary hazard whenever anybody wants to use a grinder rather then a brush is that the grinding stone or wheel becomes impregnated with zinc. Unless the wheel is changed and quarantined, zinc is inadvertantly carried into each weldment therafter and contaminates the joint. Zinc is insoluable in the molten weld puddle and can lead to centerline cracking.

Just a couple more pointers about worling with galvanized material:
Mr. Crisi is very right about staying away from the fumes when burning. The fumes will give you what we used to call "Galvanize Poison". The more fumes you breath the worse it gets. You'll become nautious, dizzy, and get a headache that will make you want to cut your head off. The thing to remember though is if you accidentally become affected, go drink a tall glass or two of milk. For some reason milk "cures" the symptoms.
To re-galvanize the weld area after weld completion "Cold-Gal" paint is a semi-effective option but "Galva-Bar" works better. "Galva-Bar" is basically a rod of zinc that you kind of braze on with a torch and wire brush. Any offshore oilfield supply house will carry it. To apply, you simply heat the base metal to the melting point of the zinc rod, smear it around the exposed steel area, then brush off the excess with a wire brush. Just don't over heat or "burn" it because it won't last as long. Be careful though because breathing the fumes will make you wish you had'nt.
Tim Gary

In the U.S, be sure you have read the OSHA Safety issues along with it's referenced sections. I'd like to see comments from some of the safety experts.

Welding over Galv material is not specifically addressed in the D1 Code. However, please note paragraph 5.15 for guidance. Essentially, any film which affects the quality of the weld must be removed.

All of the replies you received are good suggestions. AWS has issued a recommended practice on this subject which you might want to review. It is AWS D19.0-72.