Easiest option, hold down the power/screen button - a menu will pop up, one of them is Silent Mode. Click it. Done.
2nd option, search the Market for a free mute widget. You can then add the widget to any home page and click that when you want to mute or unmute your phone.
It's pretty simple, just remember to mute before your meeting. If you are like me and have set times when you want your phone silent (ie: you have a meeting every tuesday at 9am), you can download an app like Timeriffic to schedule times when your phone is on silent. It can also do much more - like turn off your wifi when you leave for work, or change your brightness at night...etc etc.