If you are shooting an important event (and ALL paid shoots are important), take some basic precautions, and bring spares.

Here is my advice:

Camera:

1. Format memory cards before you shoot.
2. Bring spare memory cards.
3. Ensure that your camera batteries are 100% charged the night before. Check this!
4. Bring spare camera batteries. At least one spare per camera.
5. Bring a charger too, just in case.
6. Bring a spare camera. This can be a starter camera or an old camera – just something you can grab when bad things happen.
7. Bring a spare lens (what if your one lens’s aperture blades gets stuck? You need an alternate!).

Flash:

1. Bring a flash – but also a spare flash of the same type. Canon TTL shooters, unless they use a 7D or a 60D, must use an IR transmitter or a 580EX flash on the camera: when that fails, your entire system is down since you can no longer remote-fire the other flashes. My 580EX II failed recently… good thing we had lots of spares.
2. Bring charged (ideally, conditioned, i.e. discharged-then-charged) NiMH flash batteries.
3. Bring lots of spares of those. No, I mean lots.
4. Also, always keep a few (8?) Alkaline batteries in your bag as emergency backup. These do not cycle as fast as NiMH batteries, but they will keep their charge for many years.

Other:

1. If you use big lights, bring more than you need. You know that your light’s flash tube will die during a shoot, not when the light is in a bag!
2. Bring a spare photographer, if you can. If you cannot, then at least bring headache and tummy-ache pills just in case.
3. Bring a charger for your cell phone.
4. Here’s one most photographers forget: Bring spares for each type of cable you use. USB if you are tethered. Remote flash cable if you use that. Flash X-type. And so on. Whatever you use – bring a spare. Cables break, or go bad, all the time.

During the shoot:

1. Change flash batteries before each segment of a shoot, even if not empty.
2. Change your camera’s memory cards every now and then in case a card malfunctions. On some high-end cameras, like my 1-series Canon bodies, you can write to two cards at once.

These simple precautions will not only save your hide sometimes – they will. But more importantly, every shoot you do will involveless stomach acid and headache. Although of course you carry pills for those.

Final bit of advice: make a personalized checklist for shoots. This too gives you peace of mind.