I want to work for NASA. As a physics major, what is the best way to get a job as a physicist at NASA?
As an upperclassman undergraduate, now is the perfect time for you to get involved in NASA projects. Do you have plans for this coming summer yet? If not, now is the time to start looking into summer internships. Internships introduce you to NASA research and provide you with knowledge, skills, and NASA contacts.
Co-ops are an excellent way to work at NASA on a more permanent basis as a student. Many co-op students have the option of becoming full-time employees after they graduate. Co-ops also provide the same benefits of an internships but with the added benefit of many terms to return to work.
Since you live in Houston, many professors at your university have contacts with JSC, and some may even work there. Have you looked at your department's research? A lot of time, professors can take on undergraduate students to work either on NASA projects or at a NASA center (since you're so close by).
Once you have NASA research experience and contacts, it will be easier for you to determine if and where within NASA you wish to work once you graduate. Many NASA employees are hired without having done any previous NASA work, but getting involved early is beneficial (and fun!).
Besides NASA itself, many NASA contractors, big and small, also offer internships. Now is a good time to start contacting the larger companies and NASA itself. The smaller companies start looking a bit later. IMHO, smaller is better, but that is just my opinion.