All of your customers are necessary. They are the reason your are in business. They are the reason you stay in business.
But some of your customers are extremely important: namely your five best customers. Take a moment and make a list of those five. They may be the biggest buyers, they may have been your first sales, they may just be the funniest clients you have. Choose them however you want, but do make a list of your top 5 customers.
Have you done it yet?
Now take a look at that list, because it is gold.
First of all, this list says a lot about you. This is a list of your ideal customers. If all of your customers were a variation of these five, you would be quite happy, I imagine. All of these customers believed in you. That means that you can compare potential customers to this list. When you see similarities, that's a good sign! This isn't a foolproof tool, of course, but it gives a nice metric you can estimate by.
The really important thing about this list, though, is that "best"-ness is often a two-way street. This means that there's an overwhelming chance that these clients would put you on a list of their best sales experiences.
While reaching out to your best customers, it may also be a good idea to try and get some testimonials, if you haven't done that yet. Don't do it unless you feel completely comfortable asking them for a testimonial.
Do you know why? Do you know the specific reasons?
If the answer is no, then this is your pot of gold. Do you know who can tell you the reasons? The people on this list. Give these customers a call to catch up with them (you have stayed in touch, right?). Find out what it is that sold them on you. Their answers may surprise you.
So, go ahead. Take another look at that list of five. There's plenty of room to learn from and improve these relationships. Get out there and start working on that. You may be shocked.