iPhone Randomly Losing Service

More and more frequently my iPhone is loosing service in areas I normally have it. I will just pick it up and notice that in the upper left hand corner it will say no service. Once I shut down and reboot, the service bars are where they should be. Any ideas on why this is happening? Is it happening to anyone else? It just ***** because I never know that it has lost service until I look at the phone and there have been several occasions where I have missed many calls or text messages-- very frustrating!
iPhone iOS 4 Verizon Wireless