Many people prune their trees and shrubs, but one thing to keep in mind while you're pruning is: Are the plants you're working on healthy?
Sometimes the peach tree you are pruning has peach leaf curl, or the Sycamore tree is riddled with Anthracnose. Or you can be helping out a friend in their yard, and they have diseased plants. Be careful!
If you suspect that the plants you are working with might not be totally healthy, then don't risk the possibility of spreading those diseases to other healthy plants.
Instead, use a 10% bleach - water solution. Keep a bucket of it with you while you prune, and if you have a really diseased plant, dip the pruners pr loppers in the bucket after EVERY cut.
I know this is labor intensive, but it really helps in reducing the spread of unwanted diseases, and in really bad cases, pests too.
Now if the tree or shrub isn't that bad, simply clean your shears when you are done, or before you move on to the next plant.
I always do this, even when I am pruning in my own yard. I just always clean up my shears and loppers, when I am done for the day, with a light bleach solution. That way when I pick them up next time, I don't even worry about spreading anything.
It's kind of a neat trick, and it will save you time and money in the long run because you will have limited the spread of disease, and will have much healthier plants overall.