For key planets that are hard to defend the space above, make sure that you have a heavy ground force. If a planet is one that is hotly contested, when captured, don't pour resources into it, but instead uses the extra credits to build an attack fleet that can secure a more defensible planet. Hold current, non-definsible planets until overwhelmed then retreat. Find key, easily defensible planets to fortify.
I haven't played the CIS, but as the Republic, my strategy was retreat and attack.
