You should be clear with workers long before the evaluation regarding the standards or rules. If they know what is actually expected, then, surely there will be no surprises throughout the assessment. 

Make certain that employee evaluations include not just your opinions and observations, but also consider those of workers as well. You can ask for their feedback on what your issues are and what you observed, and be certain to give them an opportunity to put across their ideas as well. You may sometimes uncover circumstances you were not even conscious of within the company that have longer been averting them from giving their best of capabilities.

It is always a good idea to combine negative within positive. Be certain that your assessment is properly balanced with positive feedback, accolades and praise for their achievements. Giving just negative comments will badly decrease the morale of your employees and may leave them thinking that they are not doing anything correct. You should be certain to setup a time-frame to react on almost anything that requires improvement and provide help to increase their overall performance to required levels.

You cannot make your employees happy each time you give employee evaluations, but by following above mentioned steps, you will certainly be able to improve the overall effectiveness.