When we being a project one of our first questions is 'tell us about your perfect customer'.
On of the clearest things we see watching our clients go from strength to strength is the success of those who define their target market really really tightly, then work to totally meet the needs of those target customers. The Chocolate Yogis don't try to make chocolate for everyone (I mean, who doesn't like chocolate?), they make a very tight range that satisfies the needs of vegan and dairy free customers. Duskii doesn't try to sell wetsuits at every price point or style - it has a brilliant range of gear focused at the surfing fashionista. Vintage Venus doesn't sell prints of just any art - they know they have a loyal following in vintage art prints, and prints work beautifully for them in the size and format they want.
My heart sinks when I hear a vague answer, usually followed by 'but really, I want to sell to everyone'. By trying to server everyone we end up serving no-one.
Sure we sometimes need to try a few different things to figure out where our business sweet spot is (my favorite analogy is throwing spaghetti at the fridge and seeing what sticks), but target and try, don't just constantly take a scatter gun approach.
If you have a client base already, take a hard look at it. Circle every customer who you loved (and made a profit) serving and served well - now figure out what they have in common, and how to find more of them and serve them even better.
If you don't yet have a base, make an educated guess, target your efforts at that assume client profile, and then refine it based on results.