A slogan is the essence of what makes you unique, and indispensable to the market.
How to Be Unique
With regard to being "unique," this is of course addressing why you're different from the competition.
After all, if someone is already your customer, a slogan may not impress them because they already know you.
You wouldn't brag to your spouse about how strong your calf muscles are either.
However, if someone doesn't know you, they are probably already doing business with your competitor. That means your slogan must convince them to join you and not them.
So, in order to capture the uniqueness of your products/services/company, I'd start by asking some of your customers why they use you.
Some may say some things you've already suspected, but someone may say something really genius that sums up everything that has made you successful, without you even knowing it.
For example, a few years back I had a client that needed a new brand for a company he bought (engineering equipment).
He thought the company was valuable to customers because of the easy-to-use website. However, what he found out was that it was the customer service people loved.
Most engineering firms in that industry apparently are hard to get on the phone, and our client had people 6 days a week to field calls and questions,
We quickly crafted a slogan around this benefit, and the reception was tremendous as sales surged (of course, we complimented this effort with a direct mail campaign).
How to be Indispensable
A slogan summing up the benefits of working with you are great, but a truly exceptional slogan also captures why you are indispensable to the marketplace.
This will really take some thinking, and no matter how good your slogan is, the message must be consistent in everything you do.
When Apple Inc. launched the "Think Different" slogan, there was series of ads that backed up the fact that Apple was the new contrarian on the block.
When Little Caesars launched their "pizza! pizza!" slogan, the pricing was perfectly consistent with customers' expectation that they'd be getting a second pizza for free.
So you see, merely pointing out why you're indispensable in the marketplace is easy -- living it is hard, and takes utter devotion and consistency.Don't compete — DOMINATE.