I've been playing with different guards. The one most typically seen in film representations is the middle guard, which seems like it would be a good general purpose guard, but feels to me just a touch overextended for a quick reaction to something coming from an unexpected angle. The other guard that you see a lot is the high guard, where the pommel is held at the shoulder with the blade point facing up. This is a very aggressive looking guard, but it seems to me as limiting one's attacks fairly significantly and being particularly weak against some attacks.
The guard that I've been finding particularly useful has been the hanging guard - the forward facing one on the dominant hand side. This was the most difficult guard for me to grok; it involves a bit of contortionism that twists the arms around awkwardly oddly, and it seems needlessly complex. But it makes for good defense against both low and high attacks, keeps the tip pointed at your opponent's face, and leaves you with a lot of options for attack.
On Saturday I realized that I'm quite fond of the plow guard. Very nice for getting a nice thrust in, or for feinting a thrust and then twisting the sword around their block and getting a nice cut to the ribs. I had always disliked the plow, but I'm now needing to reconsider.
But my new favorite guard is the English high guard held on the non-dominant side. It is the most Lebowski of all the guards, the most slackerly. The pommel is held at solar plexus level, with the blade resting gently against the shoulder, the tip pointing up and back. It is very restful. It's also incredibly fast, doing an overhead strike with the false edge. Did that four times in a row, and succeeded in a one-shot killing strike three times out of four. Sweet. Gives good defense against high attacks, and gives you the ability to strike fast if they try to go low.