Well, by now the engineer should have already spent some time talking to you about how you want it to sound.
Bring in some CD's with examples of the sounds you are after, but don't be unrealistic. Don't ask him to make you sound like Stevie Ray Vaughn, and then come in with a Crate amp. Stuff like that. However, a disc which has a snare drum sound you really like is very useful. Same with the kick, and really just the whole kit. It is helpful with guitars, but less so, as guitar sounds are more about what is coming out of the amp (I can fix a lot more with the drums, it just takes a while).
The most important thing is for you to PRACTICE. What I would recommend (though it is already too late) is for you to practice 6-8 hours a day for two weeks before you get into the studio. Know your songs so well, you can play them perfectly when you are a little nervous and weirded out by spending that much an hour for the studio. And when I say practice, I mean practice, not just playing them. If you are having trouble with a transition, play that transition a few times until you figure out what the problem is. Don't just play the song through, but work on the hard parts. I would say that if you are rehearsing 5 songs, you should probably not play any one song all the way through more than 2-3 times in a 6 hour rehearsal. The rest of the time should be working on the parts which are giving you trouble.
New strings are good, but not too new. Do not change your guitar strings the day of the session, as they will be stretching out, and the guitar won’t stay in tune. Change them the day before, after rehearsal, and then stretch them out. To stretch them, you need to get them in tune, and then grab each string and pull it away from the fingerboard an inch or two, and wiggle it a little. Not enough to break it, but just enough to pull out any slack at the tuners, and make up for any stretch in the string itself. Retune, and do this two or three more times. It is also a very good idea to get your instruments set up before you go in. It cost about $60-100 for a professional electric guitar set up, but it is absolutely worth it, particularly when you are going in to the studio.
If you are using a nylon string, change the strings a week ahead of time, as they take longer to stretch out. Figure on 2-3 days ahead for Bass strings, because they also take longer to stretch out than steel guitar strings.
Don't piss off the engineer, and be diplomatic, but remember it is YOUR session. He has done more sessions and knows what he is doing, so listen to his advice and give it serious consideration, but make your own decisions.
Tune the drums, sure (every one says this), but also remember to tune your guitars, dammit. This does NOT mean using a tuner, but actually using your ear to listen to the tuning. Your ear is a much more sensitive instrument for tuning than any digital tuner, and is better at hearing what SOUNDS in tune than even a strobe tuner. I get the A string on guitars in tune (to the piano or keyboards if they are being used, to a strobe tuner if they are not), and tune the rest by ear.
Five songs complete in sixteen hours is very ambitious, so be prepared to skip a few songs if you have to. Choose an order, and do them in that order. The first song to be done is not your favorite, nor the one which you wrote. The first song is the strongest song you have. The second song is the next strongest song, and so on.
In order to record that many songs in that little time, you will need to be recording almost everything but vocals in the same take (drums bass and guitar), so rehearsal is very important. If you have a great take for three of you, you can overdub the other guy, but if only one of you is together in a take, it is not going to work. Vocals will almost certainly need to be overdubbed if he is playing with the rest of you, but it is up to the engineer if that happens before the next song, or at the end of the day. I would want to do it before moving on to the next song, just because I want to make sure I have enough time to get all of the vocal tracks. However, if he can not set up for the vocal tracks without breaking down another part of the setup, then wait.
Tell your drummer to set up his drums, play a little to make sure they are right, and then get up and walk away from the set, tell the engineer every thing is in place, and then wait until the engineer tells him he can play again. There is NOTHING you can do to piss off an engineer worse than hitting the snare drum while he is positioning mics. Instant tinnitus, if you know what I mean. Read the first few days of the Mixerman diaries if you don't know what I am talking about. And make sure no one else decides this is a good time to become a drummer.
So relax and have fun (this is rock and roll, it is about fun, remember), but take the process seriously. It is fun, but it is also work. It is a lot more fun if you are getting a lot done than if you are just messing around. At the end of the session, you will be much happier if you work hard than if you play hard.
Oh yeah, and no, you will not be recording durring the mix. You will be mixing.
Light
"Cowards can never be moral."
M.K. Gandhi
(I'm such a whore). 
