Be thorough as well as systematic. For example, head to toe (toe to head in peds) rather than piecewise and scatter-like.
Always do a generalized exam, you'd be surprised at what you find. Don't document unless you did it. You'd be surprised at the patients who adamantly deny abdominal pain who have tenderness or even rigidity upon palpation.
There are excellent resources online in regards to physical assessments for both pre-hospital and medical students. It is an art and it takes time to "get your flow" down. Find someone whose clinical skills you find good/thorough/etc.. and pull tips/ideas from how they approach their assessments.