There are a number of ways, you can simply just press the volume down rocker until it's silent.
Search the App Market for "silent" and you can download a silent mode widget (there are a couple out there), which you can put on your homescreen and use as one-click toggle for silent mode.
Or you can make things really interesting by downloading the "Locale" app, which carries out certain commands on your phone (such as turn off notifications, ringers, launch/kill apps etc) based on your location and/or time! So you could make the phone automically go into silent mode as soon as you get near work! This does cost about $10 but I'm sure you'll agree it is a very clever app and will let you forget about your phone settings when going to/from work.