"Don't touch the stove, it's hot."
"Don't run in to the road, there are cars."
"Don't mess this up."
"Don't trip as you walk down the aisle with everyone watching."
We have a bias for the negative. Partly it is your brain trying to keep you alive. The amygdala has two thirds of its neurons wired for negative inputs. Studies have found that people are more likely to make a decision based on avoiding something bad than to get something good. It is also partly social conditioning too. All our lives we hear negatives. Our parents tell us not to touch the stove. Our teachers tell us not to talk in class. Our bosses tell us not to be late.
Here is the problem - after being told "don't do that" people don't actually know what they should do. You can't do a "don't." Telling someone "don't" isn't actually helpful. It is actually unclear. So what can you do?
My first suggestion would be to start a gratitude journal. This will help train your brain to start looking for the positive around you. Not only will this help you give better instructions, there are actually numerous studies that have shown people who practice gratitude are measurably happier.
The second suggestion would be to start noticing how you are stating instructions. If you are about to tell someone "don't", pause, and then tell them what you do want them to do. If a "don't" accidentally slips out then simply follow up with the "do."
"Keep away from the stove, it's hot."
"Stay on the sidewalk, there are cars in the road."
"Do your best."
"Walk carefully one step at a time down the aisle."
It will take time to change your default language from "don't" to "do" because you have been doing it for so long. It is worth the effort to make the change however because your instructions will be much easier to follow.