When it comes to improving your defense, one of the best things you can do is get familiar with every character's moveset. I'm not saying you need to spend hours in training move slaving over a hot combo, but you should know a little bit about each character's more popular moves. In particular, you should try to learn what each character's overheads and lows look like.
I'll give you an example of this as it applied to me. When Valkenhayn first came out for the 360, I had a hard time dealing with him. I didn't really know what his moves looked like so it came as a surprise when I found out the hard way that Nacht Rozen (a god damned uppercut) was an overhead that lead into at least a 4k combo. Point is, because I didn't have the character (and thus couldn't practice him) I had a really hard time against him. I had to learn everything about him by fighting other people. It wouldn've been much easier to block the mixup if I'd had the time to sit down and learn his moves.