1. Contrary to popular belief, if you go to UT, this does not mean you have an automatic "Texas connection." You have to have a separate connection, or top grades, if you want to work in Texas. I had neither, I didn't have my heart set on staying in state, and the firms could tell.
My advice: If you're below top third or so, focus on firms in markets where you do have strong connections. I got CBs in only one East Coast market where I had the strongest connection (that is, where the firms thought I was likeliest to want to live long-term). I wasted a lot of time interviewing with Texas firms who might have been interested only if I had a solid reason for wanting to work in, say, Dallas. I couldn't be happier with the way it all turned out, but I wish I'd known this going in.
2. If you've never done a callback-style interview before, get Guerrilla Tactics and read it a bunch. It helped me immensely -- mostly with nerves, and knowing what to expect and how to approach it.
3. If you can, use a real writing sample from your 1L summer job. I found out from the hiring partner at my V50 firm, which should have been out of my GPA reach, that my writing sample sufficiently impressed one of the litigation partners that he went to bat for me to get an offer. During the CB, he dissected it and gave me the kind of feedback he would give an associate, presumably to see how I would handle it. It was a little weird at the time, but it turned out to be the most important part.
4. Get lots of practice, early and often. One of the smartest things I did was sign up for a bunch of school-sponsored pre-OCI interview fairs in NYC and Washington. I knew I had little or no chance at the firms I talked with, but I got the benefit of practicing my 20-minute spiel under a wide range of conditions. (For example, I got a couple of really terrible interviews out of the way, and I got exposure to 99% of the questions interviewers would ask me throughout the process.) This experience was extremely valuable during the higher-stakes interviews in Austin with firms I really wanted. My one mock interview with CSO was useful because I got honest feedback, but it was even better to start off with a bunch of low-pressure but real screeners.