Carry extra batteries for your camera(s), and an extra storage card, or two, or three.
It sucks to be out in the field and run out of power, or run out of card storage space. This is particularly important if shooting video, which fills space rapidly, and depletes batteries quickly.
As I noted in Tip 5 “Take Lots of Photos” storage cards are dirt cheap these days. So stock up on cards and carry extras.
Batteries? Depending on the camera, not so cheap. If your camera uses a proprietary battery, it’s likely going to be expensive to get an extra one. I’ve got extra batteries for each of my DSLRs, at around $75 a pop. Not cheap, but think about it — if you take a photo ramble or photo trip, and you run out of juice, that $75 is going to look darn cheap compared to the time and cost of your venture.
And if you’re shooting for money, for a client, “sorry my battery ran out” will be a major setback to your career.
Again, if you’re shooting video, go for at least a pair of backup batteries.
You could try no-name batteries that mimic the output and dimensions of the maker’s ones. I do this for my cheaper point-and-shoot cameras. But I’m sticking with “official” manufacturer-approved ones for my DSLRs.
Don’t forget your other gear that uses batteries, too. I carry double sets of batteries for my flash units, for my remote camera trigger, etc.
One more tip within this tip:
Dedicate a drawer in your office just to batteries and chargers for all of your equipment. That way you know where everything is. Have a shoot scheduled for tomorrow? Have a charging station set up into which you can plug all your chargers, so that they are not spread out all over your house where you can forget them.
More power to you! : -)