The entire point of attracting prospects is to turn them into paying clients. If you have a prospect, it's important that you start this process straight away. As a service provider, having clients is the way you pay the bills, and therefore you should always be working on gaining the next client, even when your client roster is full.
Ideally, you want a waiting list of potential clients. That's why you continuously build a list of prospects, but don't wait until you need the client to fill a spot to start turning them into clients. Start from the moment they sign up for your list. If you do, when a spot comes open you'll have your choice of ideal clients from which to choose. You'll never be without the exact clients you want at the exact right time.
Provide all prospects with information and education freely without trying to sell to them. When someone signs up for your email list, or signs up to a webinar you're hosting, or reads your eBook, they are trusting you to deliver a certain quality. Seek to exceed their expectations in order to build the trust they'll need to further open their wallet and choose you as their service provider.
Solve Their Problems
Your audience has problems. They have issues that you can solve with your expertise. If you can describe issues that your audience has and tell how you can solve those issues, then you show them that you provide solutions for them. In other words, you don't sell a transcription service; you sell time. By transcribing for them instead of them having to do it themselves, you are giving them more time to work on more ideas and projects than they could without you.
Become a Resource
This might sound strange, but there will be prospects that you really can't help. But, you'll know someone else who can help them. It's important that you refer them to those people because 1) you've just made yourself a resource to your audience and 2) those whom you refer will remember the kindness and return the favor. Only recommend people you know will do a great job because next time that person needs work that you can do, they'll contact you again.
Provide a Fresh Perspective
Sometimes potential clients come to you already frazzled. They need so much done that they don't even know where to begin. Take charge of the call (or email) and help your potential client make a list of what needs to be done so that you can determine if you are right for the position. Help them think through the actions and work needed to reach their goal. Focusing on the goal will help your client focus too.
Finally, if you don't convert them to a paying customer at the time of the first call, ask them to be part of your email list, so that they can receive more information. In addition, whether they're part of your list or not, follow up with them in a couple of weeks or a month or two to find out how things are going for them.