Randomised controlled trials are the gold standard method for developing evidence-based medicine. Good trial design and an awareness of some potential pitfalls are likely to maximise the chances of a successful trial with a conclusion that adds meaningfully to the evidence base. This paper is aimed at people early in their research careers and focuses on some common, usually avoidable, pitfalls in trial design. The areas covered include: assessing the scientific idea; trial design; size and duration of the trial; analysis; and reporting and presentation.