From my own unofficial experience, just remember to keep their goals in mind and keep records. When you train people over a prolonged period of time you can have a tendency to hive off into irrelevancies. Make sure their goals are clear, their starting point is clear, and document their progress. Break down their major two or three goals into SMART goals. It's an acronym, stands for Specific, Measurable, Achievable, Relevant, and Timely. Specific means just that, make sure the minor goals that lead to the major ones are clear. Measurable, same thing; make sure you can measure progress against some objective scale. Achievable means don't set a goal they aren't going to reach. Relevant means it's got to add to their progress toward the big three goals. Timely means it has to be achieved in a reasonable time frame so they don't lose interest and don't feel like they're pounding away for nothing.
If I've ever seen consistent mistakes made by trainers at gyms I go to, other than the bro lore they spout, it's they rarely ask about a person's goals and really dig in to find out what they really are, and they disregard starting points and often set unachievable goals for people who really are way further out of shape than they realize and who should start slower/easier than they did.
