I'm going to go ahead and say this right off the bat: I'm a Visio junkie.

In my opinion Visio deserves a place in every administrator’s toolbox. Trust me on this - nothing makes you look less professional than sharing a diagram created on the back of a scrap piece of paper - or worse, MS Paint.

However there can be an art to using tools like Visio. Here are a few simple tips to help you create quality, professional-looking network diagrams.

Use Multiple Pages

The number one mistake in many network diagrams is an attempt to convey too much information on one page. Visio allows use of multiple pages just like an Excel workbook with many worksheets, so consider using multiple drawing sheets for different purposes. For instance, you might show your Layer 2 wiring on one page, WAN and core on another page, and your VLAN scheme on a third. You could also add pages for things like your datacenter and internet connections. Keep it simple, but use as much space as is necessary.

--> The rest is on Petri's website :

http://www.petri.co.il/network-diagrams-using-visio.htm