1. 8x10 - Your headshot MUST be on paper that measures 8x10 inches in size. Computer paper is 8.5 x 11, meaning it won't work. If you decide to print the images yourself, you'll actually have to cut each individual piece of paper to 8x10, or purchase 8x10 paper specifically. Casting directors hate being handed a standard-sized page because it won't fit in the pile of other Headshots they have to carry around. This makes them more likely to toss it.
2. Resume on back - if you're going to hand in a headshot at an audition, it needs to have your resume on the back with all your information. See this book for more info on what to include in your resume. Now remember that because your printer will be printing your resume on 8.5 by 11 paper, you'll have to cut it so it fits properly on your headshot. Then make sure to staple it at the four corners so it's securely attached.
The other three musts will come soon! Stay tuned.