Don't rush.
A method I use that has not failed me yet is to do batches of 50, so that they'll fit into the same plastic utility box. I keep my brass in a plastic bag until it gets cleaned/trimmed/ inspected. Then I size and prime the brass, putting it into the utility box upside down so I can see the primer. After all have been primed I put away the case lube and swap the die to the seating die.
I go through and put the powder in each case, flipping it rightside up so that I can see the powder. Then when all have powder I start seating bullets.
Haven't had any mistakes yet.
As mentioned, keep notes especially if you're doing load development. Utility boxes are pretty good for that, as they let you keep cartridges separate.
Try to minimize distractions, it's easy to pick up a case intending to put some powder in it, then put it down while you answer the phone, and then skip to the next case. If you don't notice the mistake later, you'll end up with a squib load. If you do need to get up and go do something else, don't leave it half done, or leave a note.
Also, keep an eye on what's near your scale. There was one time when I noticed that the base of my desk lamp had somehow gotten up close to the scale, and was making it give a constant low weight since part of the tray was being supported by the lamp. I noticed because the sound of the powder hitting the tray was different, but with certain powders even a little can do a lot.