Speak to the couple before the wedding to make sure you know exactly what they require for their wedding photography. Ideally meet them in person. This allows you build up a rapport with them too. A lot of photographers offer a pre wedding photo shoot (about an hour) for this reason, and include it in their fee.
Plenty of batteries (for your cameras and flash guns).
Plenty of memory cards. I use 32Gb cards and fill them (D600 is set to mirror the pictures across two cards in case of card failure - raw files on one card, jpegs on the other).
D600 camera body (main camera)
D200 camera body (spare). No dual card slot either, so I back up any cards asap.
Nikon 12-24 f4
Sigma 24-70 f2.8
Sigma 70-200 f2.8
Nikon 50 f1.4
Nikon 85 f1.8
Two SB-800 flashguns
Quantum battery pack (for super quick speedlight recycle)
Pocket wizard radio triggers
List of addresses (bride's preparation location, church, reception venue), inc. phone numbers. I put the zip/post codes into the sat. nav. in advance.
Phone numbers of bride and groom
List of essential photos (formal and informal, group or individuals)
Lens cloth
Blower brush
Reflector
Small step ladder
Light stands for speedlights
Tripod
Monopod
Small lastolite softbox or plastic tupperware diffuser for a speedlight
Bottle of water
Packet of sandwiches and a chocolate bar (you never know when you may next sit down to eat and drink!)
As someone else said, a small book or print-outs of poses you like.
Don't drink any alcohol, even if offered (I am 90% of the time when photographing the bride preparing, and then later at the reception).
A good night sleep the night before
I take two camera bags, the big one holds all my equipment. Then, for any venue/location take what I need for that scenario and put it into a smaller bag (this always includes a spare camera body and flash, batteries and memory cards). I leave the rest nearby in the car boot/trunk. I used to try taking everything with me at all times, but experience allows me to know what is most likely required at any time now, which lightens the load on my feet
Other things to seriously consider are:
Professional indemnity and public liability insurance
Camera equipment insurance
Even as a non-professional, you can be sued or damage your equipment.
The list above is what I take now. When I started out it was just a couple of camera bodies, a couple of lenses, a flash gun and plenty of film (I'm showing my age now!)
You also asked about flash technique for times you cannot bounce. You can get small soft diffusers for flashguns, and they do help a bit in my opinion (others may differ on that). Flash light is also quite cold in colour (compared to daylight), so I often put a 1/4 CTO gel on the flash too. But if possible, just get the flash off the camera, even if you just use a flash cable and hold the camera with your right arm and the flash out to the left with your left arm. Not easy, and pretty tricky with heavy equipment.
There are times you just have to use direct flash unfortunately. A friend photographed a wedding where the reception was held in a refurbished pig stye (I'm not kidding). Black ceiling, tiny windows, no room to move about or place light stands. Nightmare. Direct flash with a diffuser was all he could do.
Enjoy the day, weddings are fun to photograph, although busy and stressful and hard work. And that's not counting the hours spent editing the photos afterwards!
