The "best" way, is not the "free" way.
The app is called Locale, it's $10, but it's totally worth it. You can set the phone to go to quiet mode at night, or to change bacgrounds at work to something professional, or to text you when you're driving towards the grocery store so that you can get milk.
You can also try Timerrific. I've never used it, but I hear it works nicely. You can set it up so your phone changes settings at specific times.
I hate to sound like a jerk but what's wrong with turning the ringer off... it's free and it works! If you're worried about not hearing set alarms I know my Eris still sounds them when the ringer is off... not sure if the Incredible will but if the Eris does I'd assume the Incredible would too.