First, I recommend that you use WCF, .NET Remoting, or some other higher-level communication abstraction. The learning curve for “simple” sockets is nearly as high as WCF, because there are so many non-obvious pitfalls when using TCP/IP directly.
If you decide to continue down the TCP/IP path, then review my .NET TCP/IP FAQ, particularly the sections on message framing and application protocol specifications.
Also, use asynchronous socket APIs. The synchronous APIs do not scale and in some error situations may cause deadlocks. The synchronous APIs make for pretty little example code, but real-world production-quality code uses the asynchronous APIs.