A little off topic, but...
Now, I'm not saying I recommend this, but I have done a few first person books that have a shifting tense to get a particular effect. I wanted to make the story seem less like you are the main character and more like you are sitting down with the main character and they are telling you the story. So, the story is mostly in first person past tense, but when it gets to more action-ey parts it switches to first person present. Kind of like somebody at a bar describing when something crazy happened to them.
An example would be like, "It was late and I was walking home from work. It's usually easy to keep to yourself in this city, but sometimes you just walk down the wrong street at the wrong time, and when I did just that, and I knew it right away. Two guys get up from the stoop. They follow me for a few quiet minutes before they get close. My head thinks a thousand thoughts, my nerves worry themselves to shreds, but my hands, my hands just tighten up."
Heh, that's kinda off the top of my head, but just an example. It's a subtle switch, but I like how going to present makes things more urgent. And, I'm not saying that this is the best thing ever, but it's kind of a trick I like to use in first person past tense. I don't think it would work for third very well.