Any advice would be helpful...

Hey everyone,
At the end of this month I am shooting a good friends wedding, my first. Just looking for any advice anyone can offer up.

A bit about the wedding

-I am not the sole photographer. I am actually a back up. The bride saw some shots I had taken and requested that I shoot in addition to the photographer they hired.
-They don't want posed pictures, they prefer the candid shots. This is the reason they want me to take pics, because the paid photographer is telling them they have to pose.
-The photographer (brides Aunt) has been notified that I will be shooting as well, she is ok with it.
-I'm not charging them, they offered to pay me, but I told them I would do it as my gift to them.
-The wedding itself will be held outside around 6pm
-The reception is being held in a hall where lighting might be scarce.
-The wedding party will be all dressed in basic black and white
-Between the wedding and the reception they want to get pics done of them running around having fun downtown.

My gear
Rebel XSI w/ battery grip
50mm f/1.8 II (I have a hood and polarized filter for this one)
18-55mm IS kit lens
75-300mm III USM
And on loan from a friend I will have a 430EX for the day
I also have a tripod and remote that I will have with me.

Any advice would be greatly appreciated!

My general advice is leave the pro alone, if you try and shoot at the same time as them, or even stand near the pro, people will look at you instead and spoil the pro's shot. If you must, use a long lens and don't stand behind the pro, stand way the heck off to the side. Personally I don't allow other cameras along for the couple/wedding party shoot, as I know from experience it spoils my photos.

Concentrate on taking photos that wouldn't be taken otherwise, of all the friends and family. Practice in advance with the flash, and remember to use high speed sync outside. Inside ISO800/1600, lens close to wide open unless you use the 50 in which case F2.8 to make sure things are in focus, 1/50th or 1/100th. Forget the tripod and remote, you won't need them.

Also see the wedding faq.

Jamie you say it's an outdoor wedding, so in a way that makes things easier. If the reception is going to be indoors then I'd recommend going there prior and taking some test shots, just to get used to what settings you will need to use. If they are having a rehersal the day before it's great to go to that so you get an idea of what's going on during the big day.

I am going to the reception hall the day before to help set everything up, They are going to set the lights where they want them and then I am going to get someone to pose for me so I can get a feel for the room. (high white ceilings)

Jasonone, like I mentioned in my original post, I am not the only photographer, they do have one that they are paying but they have requested that I also shoot. I figure this would be a good learning experience for me seeing as I will be shadowing another photographer.