All good advice. If I may, I'd like to elaborate a little. The use of a suitable primer is essential, I use XIM in this instance. There is no better primer for adhesion on a hard, slick surface like urethane or varnish. A fair word of warning is that XIM isn't the easiest primer to use. It sets quickly and there isn't much you can do about it other than using the retarder that is made by the same company. Even that will only give you a few extra minutes of working time. The XIM should be allowed to dry for at least 2 days, then sand with 220 or finer sandpaper and clean the dust off with a tackrag.
I agree with Josephine on the oil based paint. Nothing will give you the wear resistance like a good quality oil. I use Benjamin Moore's "Dulamel" on things like this. Take the time to sand between each coat, carefully cleaning up the dust with a tackrag and strain each coat into a clean bucket using a "Paper cone strainer" that you can get from the paint store for about 15 cents each. The end result, if your carefull enough, will be a beautiful finish that will rival anything that can be produced by a cabinet manufacturer.