One thing I've found is that with comp. players is that if you have taken a planet of little stratigical value inside of their empire, they go after this planet, and if you defend, it becomes too expensive. The ideal way I've found is to scuttle the planet and keep what stuctures around the place I can. (most valuable are extractors) After this your opponet may send a ship or 2 to the planet, but they never go all out on the planet, this can later be used stragically when you front line approaches the planet, just retake it and you can now just sit and wait for an advantage such as their fleet chasing one planet leaving your target exposed.