Oh man, is it time for my callback speech?! Hooray! Here are all the things I didn't know last year about callbacks that are important.
First know that many people get one to three callbacks and this is normal. Enjoy them, do the best you can, and make the most of them. If your other ten screening interviews lead to nothing, don't get discouraged. Just keep grinding. Everyone is going to get rejection letters so do not take them personally.
When a callback call comes, they will want to talk about the dates you're available and they will often ask you again what practice areas you are interested in so they can set up interviews with people who do that work. It is good to say practice areas that the firm actually has. After screwing this up once, I learned to let calls like this go to voicemail, listen to find out what firm it was, figure out my availability and the practice areas that they actually have, go to a quiet and private place, THEN call them back (ASAP). Do NOT take a call from a number you don't recognize outdoors when jets are flying over for the airshow. Do NOT say "thank you Cravath for the callback!" in the library.
Icculus is right. You have zero commitments over the next two months that are more important than interviews and callbacks, period. I got some callbacks the week before school started. Did I schedule them as soon as possible on days when I had class? I did not, I scheduled them for later in the week when I didn't have class. Did I get offers at those firms? No I did not. Maybe I wasn't going to get an offer anyway, which is totally plausible let's be honest here, but when my classmates were scheduling their callbacks on the first available day, and I was pushing mine out, who do you think the firm is going to want? God I was stupid.
HOWEVER, there is one exception to this rule: If you get multiple callbacks out of town, try to do them on back to back days. You don't want to be flying back and forth between Chicago and your #2 city every single day in late August like I did. The firms understand that you want to fly to Atlanta or whatever only once, and they will split the cost of flying you out and putting you in a hotel for two or whatever nights. I did not know this until a recruiter asked me directly "aren't you also interviewing other places in our city? Well then can't you schedule this callback on the day after one of those?" Oh, am I supposed to do that? Yes you are supposed to do that.
Flying out of O'hare at 6am before a morning callback is painful. Fly the night before.
Finally! Even if you have ninety callbacks, you don't have an offer until you have an offer. Keep grinding *until you have an offer*. There are always stories of people who had six callbacks and got no offers from them. You might be that guy. Don't take your foot off the gas until you have an offer.