A tag line (and logo) is the most important ad you'll ever run, because if strategically sound, it can leave a lasting impression of your brand or identity. It should simply and clearly encompass the single most important impression you want your target market to retain. A lot of professionals have come up with formulas to create a tag line, which really is a series of brainstorming sessions worded in creative ways to make their claim to fame unique. Don't make the mistake of diving in without a clue! Brainstorming is critical, but don't forget about starting from square one.
Here's my opinion for those who stay up all night wondering where to start: Take a tag line, boil it down and you'll discover it closely resembles a business' position statement, which is just as critical to your identity creation. Your positioning should define in a solid, competitive statement what makes your business unique, such as, "We will position Sally's flowers as the freshest in the market." Notice Sally doesn't ramble on with, "and wonderfulest, and beautifulest, and bestest and just everything you would ever want in a flower- oh yeah, and redest too." Your brand can't be everything to everyone. Stick with one concept, one defining element, and the ultimate void in the market your product or service is filling. Whatever the "one" is, be sure it is clearly communicated in your tag line (with a clever twist). Once you have the one impression you want to make, create a logo and start brainstorming tag lines.