Not really Rocketry, I often go with a 2 tier defensive system, enough in front to make breaking the continent expensive, and a force behind it to retaliate with, noone wants to break a continent if it means he's going to lose his own bonus the next turn.
That works especially well on continents with several borders that all are connected to one single territory, Europe on the Classic map for example, Ukraine, Western and Southern Europe can be lightly defended if you have a big force on Northern Europe as a threat.