Forum responses are usually vague and along the lines of "try different viscosity ink," which, yeah, but there's tens of brands and thousands of flavors.
I put the success (thus far) down to always filling the pen all the way before flying, and also making sure the nib is pointing upwards at all times.
That being said, there's no denying just about any pen is better suited than a fountain pen for flying.
Edit: pens I've flown with - Lamy Logo and Dialog 3, various Pelikan Souveräns, the occasional Metropolitan.
Edit: Presumably obvious, but still - the reason for filling the pen all up prior to flying is that ink does not expand as air pressure decreases - unlike, well, air. If there is no air in the ink reservoir, outside pressure is more or less irrelevant.