Hangars with fighter craft will help.  Siege has light armor, and fighters are anti light, so they are optimal defensees against siege.  You also don't have to worry about blanketing your planet with turrets because fighters fly to the enemy.  I would still use some turrets, and augment it with hangars.  Maybe even keep a couple carrier cruisers in outlying systems.  Also, upgrade the planets bunkers so they can survive long enough for your defenses to kill the siege.

Also try updating your game to a newer version.  The AI has received updates since retail game release version 1.0.

Visit (www.impulsedriven.com ) and download/install IMPULSE (stardock's new digital distribution system), register your game and download game version 1.05 (the latest non-beta).

thanks,

the Monk

I usualy build about 4 repair platforms on top of each other with a jump inhibitor, about 4 hangers and some turrets on point. All of these are arranged and packed together so they all overlap, and then I park a few carriers or LRM friggets inside the group. It makes an effective defense against the AI in most cases.

Not to be condescending, don't forget to invest in upgrades that can help track the enemy's fleet. -VERY IMPORTANT- The AI just loves to flank the hell out of you when your off attacking worlds. Very annoying indeed but it sure helps alot to keep tabs on them at a distance. 

But with the defense I suggested, I managed to defend a world for 45 min from about 100 ships. My Capital ship with the long range support with 3 LRM friggates and 2 carriers managed it untill one of my main fleets could back them up.