In terms of internships and employment in general, I encourage students (high school and college) to especially maintain an active list of references. Pretty much every internship and other job position requires submission of references. Often, references are the selling point. Employers don't just use them to see if you're a good fit, but also to simply help back up what's on your resume. So, I suggest keeping something like a rollerdex of former professors, supervisors, co-workers, leads in organizational projects/community service, ect. who you've established a relationship with and can ask for a reference when needed at ready notice.
Also, when it comes to professors during college, try to continuously keep in-touch with professors you initially built a good relationship with, even if you no longer have their class. Sometimes, such professors can take it as if the student no longer cares or has "moved on and above" so to speak when that is not the case at all. Sometimes personal circumstances happen, and sometimes students just don't know such an atmosphere exists.