[Slick]

A common problem that is subtle is that it is better to use beam weapon ships over missile ships. You didn't mention what your ships were armed with so I'd mention this. The reason is that the tactic is to have your fleet stop shooting once the planet is defenseless, but before it is glassed. With missile ships, oftentimes the ship is bombarded into oblivion too easily. Also, even if the missile ships stop firing once the planet is defenseless, they may have another salvo "in the air" which will proceed to the planet and possibly destroy it.

Hope this helps.