Massed formations of Guardians? What map were you running? What race do you play typically?

Guards are a difficult thing to face when turtling, perhaps more so than siege tanks. Usually it comes down to little or no intel. If you know they're coming, then you can prepare, and be ready to set a counter-stroke for the impending next wave of cracklings.

Whatever your race, scout your enemy as often as possible. If you can preempt the move to guardians, you delay their arrival. Nothing wrong with killing some idle mutalisks and spires wherever possible.

One of the most brilliant scouting manuvers is the use of Parasites. It allows you to see what your enemy is doing without endangering your own units. Overlords and Drones are good candidates. They're cheap to replace, but still, a few seconds of hands-free intel can be vital.

If you see massed guardians, send in air-to-air units *before* they get to your base, and try to put some damage on them before they start ripping into your defenses. Focus fire is the best method, but attack-move works if your micromanagement skills aren't up to par. If they get to your base before the lot of them are destroyed, don't panic. Just keep the pressure on 'em. And while that's going on, send an attack of your own back at your enemy. If you can distract him at home, you'll find things easier to handle on your own front. Just like Carriers and Battlecruisers, the key is to see them coming before they hit.

Now that I've given some general tips, here's one last thought. Defeat is a lesson. If you know your enemy, you can prepare for him. Next time you face Netris, show him no mercy.