Course registration gets treated as an academic decision, and to a degree it is. But the thing that most often wrecks a semester is not a badly chosen subject. It is a badly shaped week: four classes stacked back to back on one day, a three hour gap that swallows every Tuesday, a nine in the morning start that you will stop attending by October. The syllabus is one variable. The timetable is the one that decides whether you show up.
Count the hours honestly before you pick anything
A credit hour is supposed to represent roughly two to three hours of work outside class for every hour in it. A fifteen credit semester is therefore not fifteen hours a week, it is closer to forty five, which is a full time job. Students routinely sign up for eighteen credits alongside a part time job and a commute, then conclude several weeks later that they are bad at time management.
Write down the hours you genuinely cannot move. Work shifts, travel, sleep, the commitments you have already made to other people. Whatever is left is your real study budget, and if the courses you have chosen require more than that, the problem is arithmetic rather than discipline. It is much cheaper to discover this at registration than in week nine.
Balance the load, not just the count
Not all credits weigh the same. Three courses with weekly problem sets, a lab and a term paper will consume a semester in a way that three lecture and exam courses will not. Look at how each course is assessed before you assume the credit count tells you the workload. Two heavy courses and two lighter ones is usually more survivable than four of similar size, because the heavy weeks fall in different places.
Deadline clustering deserves particular attention. If three of your courses set major work for the same fortnight, that fortnight will be miserable regardless of how capable you are. Course outlines usually show assessment dates before registration closes, and half an hour spent comparing them can save a genuinely bad month.
Protect the hours you actually work in
Everyone has a stretch of the day when their concentration is best, and for most people it is not the hour they are most likely to schedule a lecture into. If you think clearly in the morning, putting your hardest course at four in the afternoon costs you something real. Where the timetable gives you a choice, spend it on protecting your best hours for the work that needs them, and let the easier commitments fall into your slower stretches.
Gaps are worth planning too. An hour between classes is useful and often the most productive study hour of the day, because it is bounded and you are already on campus. Three hours is a different thing entirely, and it tends to evaporate. If the timetable forces a long gap, decide in advance what it is for.
Take the grade arithmetic into account
Course choice has a measurable effect on your average, and it is worth being clear eyed about it. A heavy credit course you are likely to struggle in pulls harder on your GPA than a light one, simply because credits weight the average. That does not mean avoiding hard courses, since a demanding transcript reads better than a cautious one, but it does mean not stacking three of them in the same term as a required course you already find difficult.
Before you commit, try the shape of the semester in our college GPA calculator. Enter the courses and credits you are considering with realistic grades rather than hopeful ones, and see what it does to your average. If the answer is uncomfortable, the fix is usually rebalancing which term a course sits in rather than dropping it altogether.
Leave yourself an exit
Know your add and drop deadlines before term starts, and put them in your calendar rather than trusting yourself to remember. The first fortnight tells you a great deal about whether a course is going to work, and acting on that information while dropping is still free is one of the few genuinely costless decisions available to a student. Waiting until week eight turns the same decision into a withdrawal on your transcript.
Finally, talk to your advisor before you finalise anything, and go with a draft rather than a blank page. Advisors are far more useful when they are reacting to a specific plan than when they are asked what you should do, and they know which combinations have quietly destroyed previous cohorts.