Susan, What is the plant?? Some may not be impressed, others may not care :o) Dan

Most plants can be unpotted and most of the soil removed. Or wash all the soil off if it's a bulb. Wrap in wet paper towels and newspaper, put in a baggie but don't seal it air tight. Then wrap the foliage and all like a bouquet of roses. Tape the bundle into the box so it doesn't move then put styrofoam peanuts or crumbled newspaper over and around it.
Send it priority mail and it should be fine.