Earn high grades in all courses. Everyone has an area of study in which he finds it easy to succeed -- which is often related to the field he finds most interesting. However, taking a range of general education courses is, typically, a required part of the college experience. Showing you can earn equally high grades in courses across the curriculum sends two messages -- that you're smart and that you're driven to succeed even when you're not working within your primary interests.
Retake classes in which you've done poorly. As hard as you try, you may not be able to avoid struggling in an occasional class. If the issue is extenuating circumstances -- such as a severe, prolonged illness or the death of a close relative -- address the instructor directly, as soon as possible. You may be able to withdraw from the course (which may negatively impact your completion rate but not your grade point average), receive extensions on deadlines or take an incomplete in the course (to finish work after the semester ends). When this isn't possible, or when the deficient grade is due to less concrete circumstances, take the course again to earn a higher grade. The original grade will, typically, still show on your transcript, but retaking the course more successfully shows the initial grade did not reflect your true ability and that you're willing to redo work to assure it is completed correctly.
Show yourself as well-rounded. In addition to taking the required introductory general education courses across the disciplines, taking courses outside of your specialty shows you have diverse interests. To best utilize these courses, you can focus this work on a second major, a specific minor or a certificate. Being able to show you have combined two different fields that are often used together in the workplace highlights your ability to multitask for companies. For example, you might be a business major and take a minor in professional writing or study engineering but add a certificate in organizational leadership or safety.
Participate in extracurricular activities -- and encourage your college to include these on transcripts or co-curricular transcripts. Most colleges have multiple types of extracurricular activities, including "Greek" organizations (including fraternities and sororities, which are often social but include a community service component), professional associations (such as honor societies for specific academic departments or student chapters of field-based groups, such as the Society of Women Engineers) and special interest clubs (such as sub-cultural, gaming, theatrical or art alliances). These groups are excellent opportunities for students to share knowledge and network. Additionally, having continued, active involvement in professional organizations, especially, can give potential job applicants a step ahead in interviews.