One of the best sources for legal writing is Irving Younger's Persuasive Writing. I have carried this book around with me since my last days of law school. Younger's words on the necessity of clarity never grow old:
There is one characteristic a lawyer's language must possess, no matter the lawyer's role or the purposes for which the lawyer deploys language, however; that characteristic is clarity. A lawyer's language should express the lawyer's thought with the pellucidity of a glass of spring water. If it does not, to that extent the lawyer's language is a confession of incompetence.
Clarity of language comes with clarity of thought. The question that precedes the writing of each word, phrase and sentence of a lawyer's language is, "What do I wish to say?" Until the precise answer to that question is known, formulation of the answer is impossible. Expression equals content, if you will, in that language can be no clearer than the thing expressed.
Observe, though, that if expression equals content, then content equals expression. One cannot think clearly unless one uses language clearly.
If my opposing lawyers kept Younger's words in mind, law life would be immeasurably easier.