The best source of info is the Forum maintained by each of the various races in which veterans provide practical info and tips for newbies, and for returnees.

• Reach the Beach Forum (Sept., New Hampshire, 200+ miles, 350 teams, lots of hills).  For RTB, someone (RunBikeSki) has prepared an "Unoffical Runners Guide" which includes ease/difficulty ratings for every leg, with a time adjustment to mile pace for each leg..  This type of info is crucial for allocating team legs and for plotting where the other van is.  Race Site.

Other tips (please add your own):

• These races have been going on for a while and most of the logistical nightmares you expect -- such as, how will I possible find the other van in a transition area? -- don't occur.
• Have each runner put a van leader's cell phone number on her number so contact can be made if there is difficlutly during the leg.  Cell reception can be spotty and it can vary depending on who one's carrier is, so the more different carriers the better.  Blackberrys and similar devices are great as well.  For all these devices, their use is very helpful especially in letting one van know how the other van is progressing so that it can be ready for the van-to-van exchange.
• Note the mileage on the odometer when leaving a transition area so that you can tell your runner and all others who pass by what the mileage is at a mid-point i a leg, generally where you're giving water.
• If you can use a Garmin, do so.  Not only does it help keep track of where you are, but it also lets you focus on your pace and not on those who may pass.  Unlike a road race where you start together, on a relay you may find someone passing you rather quickly, and it helps to know that it's that runner who's fast and not you who's slow.
• Check your leg (and the ones before it) for distance and elevation.  The official elevation map may be off in both distance and elevation.  Use something like Gmaps-Pedometer.   Knowing which legs are up and which are down is helpful for assigning legs.  Caveat emptor.
• If you lose a runner and you have to run an extra leg, hey, it happens.
• Eat and drink at every opportunity.  PB&Js are great!
• Observe the Quiet Zones.  These races live and die by the neighborhoods throough which they pass.
• Thank as many volunteers as you see on the course