there was a discussion a while back about suggestions for opera-
http://smnetwork.org/forum/index.php/topic,1249.0.html . it mentions some good ideas.
the thing that helps me the most in opera is timing out the score. sit down with your book and either the cd or if possible, during rehearsal, and mark out timings- either every 30 seconds or some people do every 15 seconds. it will make your life a lot easier later on.

i would make sure to ask the conductor or director about any cuts they are planning on making- most of the time they will take out something, and this will affect your timings.
it's also good to know how to identify where you are in the score- when someone asks you for a placement in the score you should able to tell them the page number, system number, and measure. often, the conductor will have a full orchestra score and the page numbers won't match up with everyone else's vocal score. if the score has rehearsal numbers, i usually highlight them. it might be a good idea to find out if they are planning on using rehearsal numbers.
definitely taking care of the singers in rehearsal is a good ideas. cough drops, hot water for tea, etc etc. if the stage is dusty, spraying down the air with water. it will be appreciated.
one of the big differences between theatre and opera is that usually the stage manager in opera will call the show from backstage. (obviously some people do this in theatre as well,). one of the responsibilities of the opera SM is to cue every entrance. it's helpful to have an ASM on the other side of the stage to cue those entrances. every singer entrance should be marked in your book (and your ASM's book). if you have access to a PA system to the dressing rooms, it's helpful to page the singers to backstage at 5 minutes before their entrance cue. i also like to put a warning in at 2 minutes before their entrance so that if they aren't there in time you can send someone after them.
check out the thread at the top- it has some of these suggestions and others.