Well the first key is finding a goggle that fits your face in your current helmet.
A goggle that sits well on your face will seal better and be far less prone to fog. If you get a new helmet, chances are you will have to get a new goggle.
It never ceases to amaze me how many people will get on here and say, i have brand X model y and they never fog, when i know they fog ON MY FACE, and vice versa. Everyone has different facial bone structure and the seal from goggle XY will be different on each face. Comfort is one thing, but a good dry seal is quite another... I find most people overlook this factor when dealing with goggles...
Second as was mentioned is to put effort into keeping goggles dry. Humidity, sweat, rain, snow, all can lead to fogging.
Third, if you do put goggles under the hood, make sure you completely dry them before using them again, otherwise you will fog up again. Be careful to never touch the lenses.
lastly always care spare goggles. Its just a good idea...