There is something like a de-facto standard for running IT service companies – ITIL. It is a collection of good practices for many aspects of IT service delivery – administrative organization, dealing with requirements and customer requests. I am sure you will find a ton of useful advice in those books.

It is compiled from experiences of successful service providers and kept up-to-date.

You have to provide some benefit. Most software development firms are fully capable of doing their own project management, so your value strictly as a middle-man is not particularly obvious to the client.

Instead, you need to fill some hole that isn’t already being filled. Here are some things that will help you stand out and provide benefit:

• Provide a local in-person presence to the client
  This is by far and away the single most important and powerful thing you can do. There are tens of thousands of organizations on the Internet trying to do the exact same thing as you, and the probability is that half of them (if not more) are better at it than you are. What they aren’t is local. By meeting in-person and face-to-face with the client, discussing their concerns, listening to their stories, responding to their questions, and most importantly giving them a tangible sense that you exist, you provide for a large percentage of clients a value-add that is worth, from my experience, up to around 50% of the project cost. The value of this is a personality thing that depends on the client’s own disposition. So it doesn’t always make a difference. But for those that need it, it’s a huge difference.
• Provide a guarantee
  Outsourcing is risky. There’s a very good chance that they’ll put out tens of thousands of dollars and be left with nothing of value at all. By taking on some (or all) of that risk, you can help justify your cost. Part of your job is to sort through the mountains of contractors and find the good ones. Your field experience can help decrease your own personal risk allowing you to offer a better deal to the client than he’d otherwise get. Our firm offers a solid and unconditional “satisfied or you don’t pay” guarantee. This has gained us much more than it’s lost us. On risky jobs we may double-book development and build a backup solution at the same time just to be safe. On the surface that sounds expensive, but the certainty is part of our product.
• Translate into tech-ese
  Even if developers ostensibly speak the same language as the client, they rarely understand eachother. Part of your job, a large part of your job, is to speak to the client in terms the client understands, and speak to the developer in terms the developer understands. You need to find out what the client actually wants, and get that built, even if he can’t articulate it. Developers work best with a black-and-white specification that leaves nothing implied. Clients typically lack the background knowledge to produce such a thing. That’s your job.