Going to work as a photographer obviously involves taking your camera and gear with you. I have found that I need a system to ensure everything is checked, packed and ready to go. If people are paying you to work then preparation is crucial - that said it doesn't need to be difficult. I have two sets of rituals which overlap, one is the packing of my photography bag before leaving for an assignment the other is what I do when I return to base.
I typically use two camera bodies on a daily basis with the third body as my backup. Before leaving home I always make sure my gear is clean, has fresh batteries and is time synced. On each body I put in a fresh battery, format the memory cards and set the time to the second. When I have done this I snap a picture of the world time clock, I then double check the time in the image against the cameras time-stamp. As well as making sure the time is correct this also serves as a double check that the camera is working and is not set to any strange settings. I then repeat the process for the other bodies. The beauty of taking the picture is that if I get disturbed and forget where I got to the reference image will tell me which bodies have been set. Once the bodies are all done I put freshly charged batteries in my flashes, triggers and microphones. A simple process that takes a couple of minutes in total. By making it a ritual or habit it means nothing is left to chance.
For wedding and commercial photography I have another ritual on returning home. I make a new folder in the 'working' directory on my PC labelled with the date and name of the job. Within this folder I create another folder called input and copy all of my memory cards into this. I use Photomechanic for this. Once all the cards are copied I then copy this folder to two external hard drives. I then take one of the Memory Cards from each body and put them in an envelope and write on the front the date and event. I keep this envelope away from my office until the job is processed. This might seem like overkill but it means I am extremely unlikely to lose my customers images. For press work my process is slightly different and I will go into that another time.
While the memory cards are copying and backing up I put all of my batteries on charge and put fresh memory cards into the bodies. Having a routine avoids mistakes. In the past when I have messed up it is normally because I have done something different and not followed the process!
I am not suggesting this is the best way or the only way, I just think it it is good to have a system that works for you and becomes second nature.