If you do good consulting work, a client or prospect may want you to start on a new project right away even if you're already booked. Here's how to get them to delay the start date until you are available and make them feel good about the decision.
When they press you to start sooner than you are available, ask them this question:
Would you agree that the best consultants are probably the ones that are the most sought after?
They will likely say "yes, of course." This allows you to respond by telling them something like this:
Well, that's the situation we're in right now. Our best consultants are working on a project that they need to finish up so they can give your project the full attention it deserves. We wouldn't want to put somebody less experienced on your project and I don't think you would want that either.
This explains why you are not able to start the project when they want and gives them a reason to way: they want the best consultants doing their work. Positioning your project start date this way also highlights the value of your best consultants and reduces the risk of losing the project to someone who is less experienced but available sooner.