If you use your cell phone in environments where a blaring ring isn't always the ideal, but vibrate alone doesn't always get your attention, then you'd probably do well to set your phone to vibrate first and then ring if you don't pick it up after a few seconds. This functionality is available on some phones out of the box, but many other phones (ahem, *iPhone*) don't offer this functionality. Weblog jkOnTheRun offers a simple workaround:
Customize a ringtone by adding a period of silence before the actual sound. Then load up up the custom ringtone on your iPhone. Now, make sure you set your iPhone to use the new ringtone and also to vibrate upon an incoming call. When you get dialed up, your iPhone should "play" the silenced part of your ringtone while vibrating.
Obviously this tip works with any cell phone that can vibrate and ring on incoming calls, and it's a great way to take calls discreetly while still using your ringtone when you need it. If you really want to keep your calls on the down low, try assigning an inconspicuous ringtone.