Books about trial practice (and weblogs too) give an unfair impression that trials are all about procedure and technique. Equally as important, but understandably omitted from many trial references, are discussions of the substantive law that applies to particular disputes.
Never neglect the law. In fact, start there. To prepare most effectively for trial, you must often think about it in reverse, jury instructions first. Your task at trial is not merely to adduce persuasive evidence for the judge and jury; instead, you must adduce persuasive, relevant evidence.
Especially in complex cases, the more you neglect the substantive law in your preparations, the more of a nightmare it will be to overcome your opponent's relevancy objections at trial to each varied bit of evidence. Know the law, however, you'll be prepared to explain to the judge how each particular fact fits into the larger (legal) whole.
You'll never regret a mastery of the substantive law. If lacking in your preparations, however, it will form a barrier to good trial work that no amount of procedural and technical skill will be able to overcome.
Related post: "Read Jury Instructions Early and Often."