Peaches are very suseptable to diseases which begin with poor timing of pruning associtated with cold and wet weather.  For this reason pick a beautiful day to prune them and make sure the temperature will be stable for a least the following day.  Many times a peach grower will wait to prune until the peach tree is in bloom.  For Northern states where cold winter injury can kill off most or all the crop this becomes normal. 

Clemson University Link
 
Be sure to understand the best fruiting wood is the branch which grew last year and is pencil sized in diameter. These branches should be thinned out and in general you dont need many of them.
Be sure to watch for the swollen buds or blossoms while pruning.  The larger they are the stronger they will be.  by cuting into them with a knife at the base of the bud you can see if they alive or not.  The center should be lush and have green tissue around it.  If it is dead you will see a black speck in the center.  This is the pistel and the undeveloped ovule.  Obviously if it is dark, looking like a black speck it is no longer viable.
 
University of Florida Link
 
Principal for Pruning:
Eliminate enough wood to allow light penetration to the inner scafold branches.  It is idea for wood to grow from here every summer to grow fruit the following year.  Continually remove a few viable branches if too many grow to allow others to grow the following growing season.
 
Origon University Link