The two units linked by the comma are both independent clauses. They are capable of standing alone and should be punctuated either with a period or with a semi-colon.

Correction: It was a memorable moment. I think of it often.
                                          or
                  It was a memorable moment; I think of it often.

For more on punctuation, check the LARC Electronic Writing Center grammar checklist.

Check the following resources for useful practice material: 1) Guide to Grammar and Writing, 2) Colleen Hildebrand's UCI Correction Symbols with hyperlinks.