These are some things you can do to achieve optimal results for your blocking experiments for immunohistochemistry (IHC):

• Make sure to select a blocking buffer that provides the highest signal-to-noise ratio. 
• Monitor signal strength (positive control) as well as background (negative control) using different blocking reagents. 
• Maintain optimal assay conditions by using the same blocking buffer for both, antibody dilution and blocking steps. 
• Ensure there are no substances in the blocking buffer that may interfere with a particular assay. For example, the biotin content makes non-fat dry milk inappropriate for use with any detection system that includes a biotin-binding protein.