One of the biggest issues any business would face is its unique position in the market. If you've not got a unique position, then you'll find that you've got nowhere to "stand out" of the crowd throughout your promotion.
USPs were originally defined as a position a company took to make them stand out from their competitors and make the customers of said competitors switch to them. It works as an understandable and tangible reason for people to buy your product, over that of other competing products, or as a benefit that your competitors don't offer.
For example, a bank in the UK can be promoting itself based on these propositions:
1) All call centers are in the UK—so no one is dealing with crackly lines, or broken English due to the call centers being based in a lower-cost area of the world.
2) It has no middlemen—which means there's no commissions to pay to anyone.
3) It has no shareholders, so it does not charge money for things that should be free.
That's the USP for that bank and it has resulted in many people switching to it.
The USP is anything that makes you 'different' from your peers. The only caveat to this is to make sure it's a clear difference—anything unclear, or too technical may not work, or be completely overlooked by your customers, thereby creating no reason for them to switch to you or choose you over your competitors.
Your unique selling proposition is a clearly defined statement of what your business stands for—what makes it special apart from others, and is your key to monopolizing a stake in your customers' mind. If you have not thought of a USP before, why not think through all the departments of your business e.g. finance, accounting, human resources, sales and marketing, pre- and post-sales support etc. and discover if there's anything outstanding enough to be worth telling your customers about it? Just don't be shy about promoting it.
Say, your USP could be your customer service position, whether it has to do with your response time, or your guarantee. But if your rivals are also competing on response time, one strategy is to flesh out a true instance or circumstance as a case study for prospects to refer to. By this method you're telling the story of how Mrs. Smith became very relieved that fast response time had saved her husband's life and therefore became a high-profile testimony for the hospital. People love inspiring stories because they are what make incidents sound special and unique. This would be your USP angle.
Ultimately, having a USP is nothing more than a matter of positioning perspectives. How do you want your business to be viewed by your prospects compared to others? It certainly helps that each individual's needs are already different. All you can do to the best of your abilities is to craft that position to match their needs, thereby gaining a competitive edge over other merchants/vendors. Thereafter, everything you do to reinforce the USP is to instill confidence in the prospects that your proposed solutions will work out. You will have to be fluid and flexible in your approach to reconcile with a wide range of different needs and yet align your corporate values with the common points shared by these needs in order to serve well and consistently in the business.