New Hampshire costume competitor Ryan Palmison, who holds the Guinness World Record for fastest marathon dressed as a dairy product (3:10:36 as an ice cream sundae), offers his advice on crushing it fruit of the loom style.
Modify: Anything straight out of the box is going to have hot spots. Cutting the neck hole bigger or lopping off the arms will do wonders to prevent chafing.
Make your own: Most store bought costumes are polyester—not exactly breathable. You'll stay cooler by fashioning your own from loose cotton. (if you do go the store-bought route, avoid races in summer, when temperatures swell.)
Test it: Run in your costume at least once before race day. It will help you identify and address problem areas.
Check local rules: After the Boston bombing, some events, like the Chicago Marathon, are banning costumes.
Set a record: Guinness has more than three-dozen categories for costumed running. Tip: pick a runner-friendly one. Fastest marathon dressed as a piece of fruit leaves room for creativity. Fastest marathon wearing a gas mask does not.