As a current professor who writes a lot of recommendation letters, here are my 2 cents:
- Let her know now (pref. in person) that you plan to apply to a DPT program in two years and that you would like her to write you a letter.
- Ask her what you can do to make her letter stronger. It is impossible to write a strong LOR when all you know about a student is how well they did in one class. I ask students to e-mail me a brief description of their career goals (Why PT?), as well as specific examples of things I think might be important to the admissions committee (leadership experience, eg). My strongest letters have been for students I have had in multiple classes, that assisted me with my research, and/or that served as unofficial TAs or graders in my classes. If you can't do this, then keep in contact with her, maybe once a semester. It is nice when students stop by office hours for a face-to-face chat, but e-mail updates are also helpful because I can save them and reference them later, when I write my letter.
- Send a (polite) reminder about 2-3 weeks before you want your LOR in PTCAS. Professors are notorious for getting LORs in late.
- Consider a written thank you note (via e-mail) that mentions some specific things you enjoyed about her class, or examples of how her teaching was effective. We sometimes use these notes when applying for promotion or tenure.
- Some professors might ask you to write the letter yourself, and sent it to them to submit. Try to avoid this, as it might be obvious to the admissions committee if you write your own letter.
Good luck!