I just finished 1L, working about 16-20 hours/wk in a professional job. While I found it to be manageable, and enjoyed not having to live like a poor student, here are a few cautionary pieces of advice I would offer:
1) Make sure your employer is fully on board and understands your priorities. I found that professors tend to assume that law school is your only time commitment during 1L and will liberally schedule class makeups, meetings, conferences, etc., without regard for your schedule. If your job does not give you some flexibility in scheduling around this sort of thing, you may find yourself between a rock and a hard place in terms of your time. (For the same reason, it's also helpful to let your professors and administration know of your plans- they may be accommodating if they know your situation).
2) Do not try to work during finals. For about two weeks at the end of the semester, studying for finals is a full time job. I would not want to compound that stress by trying to squeeze in work time.
3) You will probably have to sacrifice a lot of the non-academic activities that come with law school. Whether it's happy hour, intramural sports or other extracurriculars, the decision to work during 1L means that you will not have time for a lot of the events that make law school fun. The hardest part for me was having to turn down a lot of invitations to do fun things with fun people because I just didn't have time.
4) Make law school your first priority and don't lose sight of it. I made myself available to my employer by cell phone and email all the time, but I also let them know that I would not always get right back to them if I was busy with school. Depending on your job, there may be some pressure to put it ahead of your school work, but no part time job is worth sacrificing your academic performance.
This is likely to get me flamed, but the fact is that most law students have way more free time than they like to admit. If you have good time management skills and an understanding employer, there is no reason that you cannot keep working and be successful in law school with a little extra effort.