Such great advice so far, everyone! And congrats to all of your successes.
I will too echo the "fit, fit, fit" spiel. I am going to the school that was my best fit, even though I was anxious because of the location (nearly 1000 miles from my home state). However, now that I'm in and seeing stuff actually materialize, I couldn't be happier.
Don't be afraid to go to a school that isn't first tier. The place needs to be right for you and what you want to do. If you find that you need more, or you're not comfortable going somewhere... Defer if you can, or apply to terminal MAs, as well.
Study for your GREs. While I did above average after only 2 days of cramming, it made me realize I could do so much more with myself and my scores. I didn't retake it, but probably should have. If you have any doubts, study and take early, and then retake. It's a pain in the neck, but it will help you.
If your GPA isn't so hot, it WILL affect you. It isn't everything, but there's a reason for this-- and a stunningly high GRE can offset this (see "hot tip" below).
Try and do research if you can help it. Even if it's a small project or a one-year thesis-- go for it. It will make your application so much stronger.
Revise revise revise--- your application, your SOP, your diversity statements (if application), your writing samples... Everything. Turn in something you are proud of, both in terms of minutiae and actual content. A passionate letter and a creative, well-written research paper/thesis both go a long way with adcoms, especially if your program has some cool fields.
Hot tip. I've heard of a few big schools having points cut-offs. Not necessarily in terms of a minimum GPA or GRE, but a formula that combines those two (usually GPA*GRE > x). While that information is usually kept under wraps, some schools are transparent about it. If you can't get over that hurdle, then you're going to have a really hard time pushing your app forward, even if you have gobs of research experience and beautiful letters from professors who really know you (this happened to me).
That said, even if you aren't totally there for a school's PhD requirements, you could get into an MA or MS in their department if it's offered. From there, you might be able to transition if they like you. If this happens, don't be afraid to consider it. It'd be a struggle for awhile, but if you really want this, then hopefully it will be worth it. Remember this: Even if you don't get in to the PhD right away, but you are offered an MA/MS, someone there really wants you. You wouldn't have had been considered for the master's program.
Finally, don't give up hope, but be realistic. It's easy to get discouraged and even depressed, but remember that there's always next year. The financial hit sucks, and so does the sting of rejection, but it's really competitive out there right now... Anything can happen.
Good luck everyone, and I look forward to seeing the successes of this round of applicants!