I am getting ready to re-apply. Should I use my letters of recommendation from last year, or is it common to obtain new letters? What if I am re-applying to some of the same schools? Just wondering how Admission committees evaluate re-applicants.
Depends on how strong you feel your current letters are.
Evaluate the possible reasons that you may not have been accepted this year.
I think that a total application "make over" can be a great way to show change/improvement from one application cycle to the next. You can always consider keeping some of your LOR's that you might suspect are stronger and getting rid of some that may not be as good.
I would aim to get at least 1 new LOR....Re-applicant here and I was recently accepted, and I think the key is DO SOMETHING DIFFERENT. If it didn't work the first time, then you have to make changes somewhere. Good Luck!