I guess a discussion of what to wear to your interviews is a little overdue. I am incompetent to discuss women's clothing, so I'll just invite people who know what they're talking about to leave advice in the comment thread. Also, here's an old post from P.G.O.A.T. One thing I will say is that it seems to me that women have more options than men. But that's not obviously a good thing for women.
For the dudes: you should wear either a suit or a jacket/tie/nice pants combination. There are always the objections about how paying attention to how people dress is superficial and stupid and not at all important. To those people I say, adults display their respect for their interviewers and their interest in the job by dressing up for interviews. Adults who don't dress up for interviews display a lack of interest and respect. Aristotle says we are political animals. Be political.
You can, of course, spend as much as you want on a suit. Macy's is a good bet for expensive suits. Men's Wearhouse is cheaper, but I find that their suits look exactly like they came from Men's Wearhouse. Nevertheless, my suit is from Men's Wearhouse. Make sure it fits you.
Finally, a tip. Wear your interview outfit as much as possible between now and the APA. Practice. I always see guys around the hotel who are obviously extraordinarily uncomfortable in their weird, unfamiliar clothes. The key to being comfortable in a suit is being accustomed to wearing it, and the only way to do that is to wear it. So wear it. Now.