How do I balance description and evaluation when answering an essay based question?

Firstly, the amount of description and evaluation expected of you will depend upon the wording of the question. Once you have considered what is expected of you, you can proceed confidently to answer the question.
I think that it is always best to hit your descriptive points first (which will be the easiest marks to get). If you are describing Classical Conditioning, you will be able to get your marks for explaining the principles of the theory, as well as how the theory was formed (by briefly describing Pavlov's dogs. For example, you would get a mark for saying that Classical Conditioning is 'learning through association, wherein we are conditioned to respond to a stimulus in a way that we normally would not.'
When evaluating we can break our arguments down into the following categories - Supporting Studies, Weaknesses, Alternative explanations, Usefulness. For example, when providing an evaluation of Classical Conditioning you could say that 'Watson and Rayner classically conditioned a baby to have a fear of white rats which supports this theory of learning. However, the experiment was done in a lab and so had poor ecological validity.' This type of response would be likely to get one or two marks (for not much writing!)