Like many others here, I have used Visio with some success. However, I should also point out another tool that I used way before Visio was around. I see now that it is still available and at a lower cost than Visio. It is RFFlow -- see http://www.rff.com. I have not used it since Visio came on the scene, but it looks like they've kept the product up to date with current methodologies.