The only interview I did was with Air Wisconsin (a.k.a. AWAC), so my experience is limited to that. I may have over-prepped, but if you've been out of the loop for a while, it will all be useful.
1. Sheppard Interview prep. It's a bank of ATP style questions (cost 50 bucks I think). The first thing we did at the interview was to take a short knowledge test. The questions were well covered by the prep. I only had time to go through the prep question bank once and that was enough.
2. Everything Explained for the airline pilot. Good book with lots of stuff to brush up on that you once knew and forgot years ago. I got it and browsed through it on a somewhat regular basis.
3. Ever use Jepps? I hadn't, so I got on the web and looked at whatever Jepp gouge I could find. That helped out a bit as they took out the jepps at the interview and asked a few questions on symbology. They also asked me to brief an approach. Not too hard, but having prepped I felt better and certainly did better.
4. Interview prep will be recommended by most, but not necessary in my opinion for a regional interview. Nevertheless, I had at least reviewed the Emerald Coast Audio Prep figuring that I'd need to do it for an eventual majors interview anyhow. So, no harm in doing that early.
5. If you have an extensive military flying record (and most of us do who have been in to retirement), you don't have to hide the fact that the regional you are interviewing at is not your final destination.
At my interview the first question I took had them holding up my resume and saying "What are you doing here?". I gave an honest answer and they were good with that. There will be some HR style questions, but for me the majority were more technical items- they were just making sure you actually sounded like a pilot and could figure out what taxiway markings meant.
In summary, if you want to fly for the majors (and we all do), you'll have to be ready for that interview some day. So, any practice you do now for a regional interview will serve you well both there and be an investment toward the final future goal. Don't stress it, but realize that no prep is wasted either.
Lastly, WFFF "ride reports" feature interview gouge for everybody. It was very helpful and when I was done I was a good citizen and wrote up my own interview experience while it was fresh in my mind. Go to willflyforfood.com and it'll all be there.
Hope that helps!