Check out my race report here:
http://gorunninghamsgo.blogspot.com/2010/03/one-night-last-july-jon-was.html
1) Bring along a small digital camera (or the disposable they give you in your goodie bag) and stop for pictures with characters along the way. My fiance is a more serious runner, so he ran for time, whereas I was a beginner and ran to finish. He was happy to see all of my pictures in the end because there was a lot of stuff he missed along the way.
2) If it's not overcast, the run back toward Epcot right after the Magic Kingdom is pretty much full sun. Around mile 8 or so, I finally shed my long sleeved top for my tank top, and I was happy to have my sunglasses. (FYI, my running sunglasses are from Target and cost somewhere around $17 I believe)
3) Dress up if you want! I didn't do a full costume because I know it would have irritated me, but I did wear a running skirt (that I'd run in many times before) and I wore a tiara and put ribbons in my hair. The fun of a Disney race is really getting to have fun with the theme. Just make sure you run in it beforehand so you know you can deal with it on race day. (And no, my tiara did not bounce around at all. It was a small one on a comb that went snugly into my hair on the top of my head)
4) If you can hold it, wait for bathrooms inside the MK or after, which should have significantly less of a line. The porta potties within miles 1-3 have lines and you'll have to stop and wait in them. I only had to go once, and it was around mile 4 I believe. No line!
5) Soak it all in. The Princess was my first half, and of the two I did last year it was definitely my favorite. The atmosphere in a Disney race is like no other, especially for Disney lovers. The crowd really builds up around the TTC and along Main Street, USA. Both of those points were so emotional for me. Hearing the crowd cheering and screaming really put me in the moment and I kept thinking, "Wow, I can't believe I'm running a half marathon and I'm doing it in Disney World!"
6) Try to thank the volunteers along the way. They work hard putting on a great race for you!
7) If you can afford it, buy the pictures. I bought the digital download of all of my pictures for $75 or whatever obscene price it was. But it was totally worth it. I love to look at the pictures of myself running through the race and crossing the finish line, and to me, they were worth every penny. You could even say that a tshirt may wear out over time, but a photograph is 'forever'.
8) And lastly, keep in mind that the apparel at the expo will likely sell out of sizes quickly, so get what you want when you see it. If you go home after the race and wish you'd bought a piece of race merchandise, call the Disney Parks merchandise line and see if they have your size. I ordered the Princess tank top after the fact (though they were out of Med and I had to buy a Lg) because I had decided not to buy any shirts at the expo and I wished I had. Keep in mind the shipping is like, $10 or $12 to do this.