There's a great article in the March-April 2010 issue of BYO on this exact subject.
Basically, you need to think of your foam as millions of little ballons. These balloons have beer around them, but all of these balloons are constantly being blown up. The layer of beer around them doesn't grow with the balloon, but instead stretchs around it until it's eventually too thin to contain the balloon. When there's no longer any beer around the balloon, it pops. Once one pops, it sends off a chain reaction to the other balloons.
Kind of an odd way to think of foam, but it's simpler than listing all the equations in that article.
Foam is effected by so many forces, some controlable and some not (gravity for instance), that it's hard to pinpoint your problem, if you even have one.
As the article says... "A beer is often said to have "good head-retention" if the foam generated by the initial pour of beer has a half-life of 60-90 seconds." So it sounds like you're head is perfectly fine.
By the way...dishwashers are the devil for beer glasses. They leave residue on the glasses from the soap. I clean mine with hot water and let them air dry.
