First, make sure you understand the official runDisney costume rules. Check out the runDisney website for the official costume and safety guidelines.
A lot of these rules can be subjective based on the cast member that sees you walking through the gates. It's a very touchy subject! My favorite running podcast, Not Real Runners, does a really great job explaining this in their Episode 35. It's worth a listen if you're feeling like your costume may be teetering on the line of acceptable according to the official rules.
Now onto my top 5 costume tips and tricks:
Plan your costume around your most comfortable and runner friendly base layers. Your legs, feet... mind... are all going to be hurting - don't top it off with uncomfortable clothing. Use pieces you already trust, or make sure to test them out ahead of time. Double check for chafe protection and when in doubt, Body Glide it out!
Side note: For Walt Disney World races in Florida, bring both hot and cold weather layers. The weather changes day to day. It snowed one year... SNOWED! And the next day it was hot and 90% humidity.
This is a great opportunity to try different and off-the-wall outfits! The standard main characters will always be present at these races, so maybe try a more unknown character or perhaps a mashup. I promise you, the reactions will be priceless and you'll feel like a rockstar!
Dress with friends
Group costumes always have a great impact! More people dressed up will make your theme more recognizable and fun!
Don't forget the back
Yes, you will have the pictures of your front to post on the internet later, but all of your fellow runners on the course are mostly going to see your back as you speed on past. If you want people to know who/what you are, make sure you're recognizable from behind!
Foam and Felt are your Friends
They come in so many colors, thicknesses, glitters, etc. that you can pretty much make anything to attach to your costume. Not to mention, they are super light weight which is important when running long distances!
Side note: Bring extra safety pins to make any costume adjustments in the middle of the race. You don't want to have to ditch an important piece of your costume because the glue didn't stick :(