For most employees, the script, the mission, the "what-we-stand-for" has been created by someone else, they just need to get behind it. (Or, if they're further up the ladder they get to be part of formulating those things in a room with others, maybe some experts thrown in the mix to help.)
It's easy when one works for a big organization--part of an established culture, attached to a set of pre-defined ideas--to feel validated, justified, on the inside of the right information. In this sort of role, people gain authority by association with the company they work for.
But when you work alone or run a small business you are inventing your own authority each day. No wonder it's hard to do your elevator pitch when it's something you crafted yourself, alone in a room with your computer.
What can the solopreneur or the individal worker do to counteract this disadvantage?
Start by recognizing your own authority. There's a reason you do what you're doing. Get behind yourself.
Gather your own "room with others," whether that means inviting voices to the table, or finding a place where people gather and going to them. And when you do, don't be shy to share your plans and ideas. Each conversation and bit of input can be a valuable contribution to your enhanced ability to stand with both feet firmly planted in your own position.
Finally, recognize that every person or client you work with is a building block for your own association. They recognize your authority. You should, too.