Jerome,
How many ways of bending light to your wishes do you have? A couple of extra flashes (that either speak to each other or have radio control) can make a world of difference if you know how to use them together.
I also would not be without a great reflector, which IMO is more important than a flash, even.
I use a California Sunbounce 4' x 6'; I can actually handle that myself usually, but it's better with 2people. Don't think you can use those things without sandbags if you're not holding onto it; they're giant sails. BTW--the cheap reflectors tend to break. There's nothing worse than trying to look professional while those things are taking chunks of flesh out of your arm
I also wouldn't be without a system to hold and categorise cards used and unused. I use the think tank photo pixel pocket rockets... one for CF and one for SD cards.
Lastly, if you're going to do this, I personally like primes and carry cameras with me (or have an assistant carry the lens bag); I don't ever put them down. So a bag like a Tamrack Velocity 9 is worth its weight in gold; I can put 3 cameras on me and carry the film camera in the bag with all the glass.
Not photographic, but I always carry Advil, duct tape and a small first aid kit

Not always for me, either.
Good luck! Having another shooter will definitely help you deliver more value too, as long as you're comfortable in their abilities.